diff --git a/src/api/custom_syntax.rs b/src/api/custom_syntax.rs
index 1a557b43..7cc299c1 100644
--- a/src/api/custom_syntax.rs
+++ b/src/api/custom_syntax.rs
@@ -12,7 +12,7 @@ use crate::{
 };
 #[cfg(feature = "no_std")]
 use std::prelude::v1::*;
-use std::{borrow::Borrow, ops::Deref};
+use std::{borrow::Borrow, collections::BTreeMap, ops::Deref};
 
 /// Collection of special markers for custom syntax definition.
 pub mod markers {
@@ -257,19 +257,29 @@ impl Engine {
                 // Standard or reserved keyword/symbol not in first position
                 _ if !segments.is_empty() && token.is_some() => {
                     // Make it a custom keyword/symbol if it is disabled or reserved
-                    if ((!self.disabled_symbols.is_empty() && self.disabled_symbols.contains(s))
+                    if (self
+                        .disabled_symbols
+                        .as_ref()
+                        .map_or(false, |m| m.contains(s))
                         || token.map_or(false, |v| v.is_reserved()))
-                        && (self.custom_keywords.is_empty()
-                            || !self.custom_keywords.contains_key(s))
+                        && !self
+                            .custom_keywords
+                            .as_ref()
+                            .map_or(false, |m| m.contains_key(s))
                     {
-                        self.custom_keywords.insert(s.into(), None);
+                        self.custom_keywords
+                            .get_or_insert_with(|| BTreeMap::new().into())
+                            .insert(s.into(), None);
                     }
                     s.into()
                 }
                 // Standard keyword in first position but not disabled
                 _ if segments.is_empty()
                     && token.as_ref().map_or(false, Token::is_standard_keyword)
-                    && (self.disabled_symbols.is_empty() || !self.disabled_symbols.contains(s)) =>
+                    && !self
+                        .disabled_symbols
+                        .as_ref()
+                        .map_or(false, |m| m.contains(s)) =>
                 {
                     return Err(LexError::ImproperSymbol(
                         s.to_string(),
@@ -283,12 +293,19 @@ impl Engine {
                 // Identifier in first position
                 _ if segments.is_empty() && is_valid_identifier(s) => {
                     // Make it a custom keyword/symbol if it is disabled or reserved
-                    if (!self.disabled_symbols.is_empty() && self.disabled_symbols.contains(s))
-                        || token.map_or(false, |v| v.is_reserved())
-                            && self.custom_keywords.is_empty()
-                        || !self.custom_keywords.contains_key(s)
+                    if self
+                        .disabled_symbols
+                        .as_ref()
+                        .map_or(false, |m| m.contains(s))
+                        || (token.map_or(false, |v| v.is_reserved())
+                            && !self
+                                .custom_keywords
+                                .as_ref()
+                                .map_or(false, |m| m.contains_key(s)))
                     {
-                        self.custom_keywords.insert(s.into(), None);
+                        self.custom_keywords
+                            .get_or_insert_with(|| BTreeMap::new().into())
+                            .insert(s.into(), None);
                     }
                     s.into()
                 }
@@ -373,14 +390,16 @@ impl Engine {
         scope_may_be_changed: bool,
         func: impl Fn(&mut EvalContext, &[Expression], &Dynamic) -> RhaiResult + SendSync + 'static,
     ) -> &mut Self {
-        self.custom_syntax.insert(
-            key.into(),
-            CustomSyntax {
-                parse: Box::new(parse),
-                func: Box::new(func),
-                scope_may_be_changed,
-            },
-        );
+        self.custom_syntax
+            .get_or_insert_with(|| BTreeMap::new().into())
+            .insert(
+                key.into(),
+                CustomSyntax {
+                    parse: Box::new(parse),
+                    func: Box::new(func),
+                    scope_may_be_changed,
+                },
+            );
         self
     }
 }
diff --git a/src/api/definitions/mod.rs b/src/api/definitions/mod.rs
index 0c6a84b1..e09159ef 100644
--- a/src/api/definitions/mod.rs
+++ b/src/api/definitions/mod.rs
@@ -369,6 +369,7 @@ impl Definitions<'_> {
             .engine
             .global_sub_modules
             .iter()
+            .flat_map(|m| m.iter())
             .map(move |(name, module)| {
                 (
                     name.to_string(),
@@ -445,13 +446,16 @@ impl Module {
             first = false;
 
             if f.access != FnAccess::Private {
-                #[cfg(not(feature = "no_custom_syntax"))]
-                let operator = def.engine.custom_keywords.contains_key(f.name.as_str())
-                    || (!f.name.contains('$') && !is_valid_function_name(f.name.as_str()));
-
-                #[cfg(feature = "no_custom_syntax")]
                 let operator = !f.name.contains('$') && !is_valid_function_name(&f.name);
 
+                #[cfg(not(feature = "no_custom_syntax"))]
+                let operator = operator
+                    || def
+                        .engine
+                        .custom_keywords
+                        .as_ref()
+                        .map_or(false, |m| m.contains_key(f.name.as_str()));
+
                 f.write_definition(writer, def, operator)?;
             }
         }
diff --git a/src/api/events.rs b/src/api/events.rs
index a44ee2a4..eff11efd 100644
--- a/src/api/events.rs
+++ b/src/api/events.rs
@@ -360,7 +360,7 @@ impl Engine {
             + SendSync
             + 'static,
     ) -> &mut Self {
-        self.debugger = Some((Box::new(init), Box::new(callback)));
+        self.debugger = Some(Box::new((Box::new(init), Box::new(callback))));
         self
     }
 }
diff --git a/src/api/mod.rs b/src/api/mod.rs
index 6f1ad649..6b48c744 100644
--- a/src/api/mod.rs
+++ b/src/api/mod.rs
@@ -35,6 +35,7 @@ pub mod definitions;
 
 use crate::{Dynamic, Engine, Identifier};
 
+use std::collections::BTreeSet;
 #[cfg(feature = "no_std")]
 use std::prelude::v1::*;
 
@@ -107,7 +108,9 @@ impl Engine {
     /// ```
     #[inline(always)]
     pub fn disable_symbol(&mut self, symbol: impl Into<Identifier>) -> &mut Self {
-        self.disabled_symbols.insert(symbol.into());
+        self.disabled_symbols
+            .get_or_insert_with(|| BTreeSet::new().into())
+            .insert(symbol.into());
         self
     }
 
@@ -163,18 +166,31 @@ impl Engine {
             // Active standard keywords cannot be made custom
             // Disabled keywords are OK
             Some(token) if token.is_standard_keyword() => {
-                if !self.disabled_symbols.contains(token.literal_syntax()) {
+                if !self
+                    .disabled_symbols
+                    .as_ref()
+                    .map_or(false, |m| m.contains(token.literal_syntax()))
+                {
                     return Err(format!("'{keyword}' is a reserved keyword"));
                 }
             }
             // Active standard symbols cannot be made custom
             Some(token) if token.is_standard_symbol() => {
-                if !self.disabled_symbols.contains(token.literal_syntax()) {
+                if !self
+                    .disabled_symbols
+                    .as_ref()
+                    .map_or(false, |m| m.contains(token.literal_syntax()))
+                {
                     return Err(format!("'{keyword}' is a reserved operator"));
                 }
             }
             // Active standard symbols cannot be made custom
-            Some(token) if !self.disabled_symbols.contains(token.literal_syntax()) => {
+            Some(token)
+                if !self
+                    .disabled_symbols
+                    .as_ref()
+                    .map_or(false, |m| m.contains(token.literal_syntax())) =>
+            {
                 return Err(format!("'{keyword}' is a reserved symbol"))
             }
             // Disabled symbols are OK
@@ -183,6 +199,7 @@ impl Engine {
 
         // Add to custom keywords
         self.custom_keywords
+            .get_or_insert_with(|| std::collections::BTreeMap::new().into())
             .insert(keyword.into(), Some(precedence));
 
         Ok(self)
diff --git a/src/api/register.rs b/src/api/register.rs
index 7f590266..518c524b 100644
--- a/src/api/register.rs
+++ b/src/api/register.rs
@@ -683,8 +683,10 @@ impl Engine {
         name: impl AsRef<str>,
         module: SharedModule,
     ) -> &mut Self {
+        use std::collections::BTreeMap;
+
         fn register_static_module_raw(
-            root: &mut std::collections::BTreeMap<Identifier, SharedModule>,
+            root: &mut BTreeMap<Identifier, SharedModule>,
             name: &str,
             module: SharedModule,
         ) {
@@ -717,7 +719,12 @@ impl Engine {
             }
         }
 
-        register_static_module_raw(&mut self.global_sub_modules, name.as_ref(), module);
+        register_static_module_raw(
+            self.global_sub_modules
+                .get_or_insert_with(|| BTreeMap::new().into()),
+            name.as_ref(),
+            module,
+        );
         self
     }
     /// _(metadata)_ Generate a list of all registered functions.
@@ -737,7 +744,7 @@ impl Engine {
         signatures.extend(self.global_namespace().gen_fn_signatures());
 
         #[cfg(not(feature = "no_module"))]
-        for (name, m) in &self.global_sub_modules {
+        for (name, m) in self.global_sub_modules.iter().flat_map(|m| m.iter()) {
             signatures.extend(m.gen_fn_signatures().map(|f| format!("{name}::{f}")));
         }
 
diff --git a/src/api/type_names.rs b/src/api/type_names.rs
index 07434e4d..258991ab 100644
--- a/src/api/type_names.rs
+++ b/src/api/type_names.rs
@@ -205,6 +205,7 @@ impl Engine {
                 return self
                     .global_sub_modules
                     .iter()
+                    .flat_map(|m| m.iter())
                     .find_map(|(_, m)| m.get_custom_type(name));
                 #[cfg(feature = "no_module")]
                 return None;
@@ -238,6 +239,7 @@ impl Engine {
                 return self
                     .global_sub_modules
                     .iter()
+                    .flat_map(|m| m.iter())
                     .find_map(|(_, m)| m.get_custom_type(name));
                 #[cfg(feature = "no_module")]
                 return None;
diff --git a/src/engine.rs b/src/engine.rs
index a17fb24d..82993f46 100644
--- a/src/engine.rs
+++ b/src/engine.rs
@@ -96,7 +96,8 @@ pub struct Engine {
     pub(crate) global_modules: StaticVec<SharedModule>,
     /// A collection of all sub-modules directly loaded into the Engine.
     #[cfg(not(feature = "no_module"))]
-    pub(crate) global_sub_modules: std::collections::BTreeMap<Identifier, SharedModule>,
+    pub(crate) global_sub_modules:
+        Option<Box<std::collections::BTreeMap<Identifier, SharedModule>>>,
 
     /// A module resolution service.
     #[cfg(not(feature = "no_module"))]
@@ -106,14 +107,16 @@ pub struct Engine {
     pub(crate) interned_strings: Locked<StringsInterner>,
 
     /// A set of symbols to disable.
-    pub(crate) disabled_symbols: BTreeSet<Identifier>,
+    pub(crate) disabled_symbols: Option<Box<BTreeSet<Identifier>>>,
     /// A map containing custom keywords and precedence to recognize.
     #[cfg(not(feature = "no_custom_syntax"))]
-    pub(crate) custom_keywords: std::collections::BTreeMap<Identifier, Option<Precedence>>,
+    pub(crate) custom_keywords:
+        Option<Box<std::collections::BTreeMap<Identifier, Option<Precedence>>>>,
     /// Custom syntax.
     #[cfg(not(feature = "no_custom_syntax"))]
-    pub(crate) custom_syntax:
-        std::collections::BTreeMap<Identifier, crate::api::custom_syntax::CustomSyntax>,
+    pub(crate) custom_syntax: Option<
+        Box<std::collections::BTreeMap<Identifier, crate::api::custom_syntax::CustomSyntax>>,
+    >,
     /// Callback closure for filtering variable definition.
     pub(crate) def_var_filter: Option<Box<OnDefVarCallback>>,
     /// Callback closure for resolving variable access.
@@ -144,10 +147,12 @@ pub struct Engine {
 
     /// Callback closure for debugging.
     #[cfg(feature = "debugging")]
-    pub(crate) debugger: Option<(
-        Box<crate::eval::OnDebuggingInit>,
-        Box<crate::eval::OnDebuggerCallback>,
-    )>,
+    pub(crate) debugger: Option<
+        Box<(
+            Box<crate::eval::OnDebuggingInit>,
+            Box<crate::eval::OnDebuggerCallback>,
+        )>,
+    >,
 }
 
 impl fmt::Debug for Engine {
@@ -168,7 +173,8 @@ impl fmt::Debug for Engine {
             "custom_syntax",
             &self
                 .custom_syntax
-                .keys()
+                .iter()
+                .flat_map(|m| m.keys())
                 .map(crate::SmartString::as_str)
                 .collect::<String>(),
         );
@@ -264,17 +270,17 @@ impl Engine {
             global_modules: StaticVec::new_const(),
 
             #[cfg(not(feature = "no_module"))]
-            global_sub_modules: std::collections::BTreeMap::new(),
+            global_sub_modules: None,
 
             #[cfg(not(feature = "no_module"))]
             module_resolver: Box::new(crate::module::resolvers::DummyModuleResolver::new()),
 
             interned_strings: StringsInterner::new().into(),
-            disabled_symbols: BTreeSet::new(),
+            disabled_symbols: None,
             #[cfg(not(feature = "no_custom_syntax"))]
-            custom_keywords: std::collections::BTreeMap::new(),
+            custom_keywords: None,
             #[cfg(not(feature = "no_custom_syntax"))]
-            custom_syntax: std::collections::BTreeMap::new(),
+            custom_syntax: None,
 
             def_var_filter: None,
             resolve_var: None,
diff --git a/src/eval/debugger.rs b/src/eval/debugger.rs
index 92f6bc0a..93663293 100644
--- a/src/eval/debugger.rs
+++ b/src/eval/debugger.rs
@@ -512,7 +512,9 @@ impl Engine {
         let src = src.as_ref().map(|s| s.as_str());
         let context = crate::EvalContext::new(self, global, caches, scope, this_ptr);
 
-        if let Some((.., ref on_debugger)) = self.debugger {
+        if let Some(ref x) = self.debugger {
+            let (.., ref on_debugger) = **x;
+
             let command = on_debugger(context, event, node, src, node.position())?;
 
             match command {
diff --git a/src/eval/expr.rs b/src/eval/expr.rs
index 0ee555f4..a7c4a260 100644
--- a/src/eval/expr.rs
+++ b/src/eval/expr.rs
@@ -36,7 +36,12 @@ impl Engine {
 
         // Do a text-match search if the index doesn't work
         global.find_import(root).map_or_else(
-            || self.global_sub_modules.get(root).cloned(),
+            || {
+                self.global_sub_modules
+                    .as_ref()
+                    .and_then(|m| m.get(root))
+                    .cloned()
+            },
             |offset| global.get_shared_import(offset),
         )
     }
@@ -401,13 +406,17 @@ impl Engine {
                 // The first token acts as the custom syntax's key
                 let key_token = custom.tokens.first().unwrap();
                 // The key should exist, unless the AST is compiled in a different Engine
-                let custom_def = self.custom_syntax.get(key_token.as_str()).ok_or_else(|| {
-                    Box::new(ERR::ErrorCustomSyntax(
-                        format!("Invalid custom syntax prefix: {key_token}"),
-                        custom.tokens.iter().map(<_>::to_string).collect(),
-                        *pos,
-                    ))
-                })?;
+                let custom_def = self
+                    .custom_syntax
+                    .as_ref()
+                    .and_then(|m| m.get(key_token.as_str()))
+                    .ok_or_else(|| {
+                        Box::new(ERR::ErrorCustomSyntax(
+                            format!("Invalid custom syntax prefix: {key_token}"),
+                            custom.tokens.iter().map(<_>::to_string).collect(),
+                            *pos,
+                        ))
+                    })?;
                 let mut context = EvalContext::new(self, global, caches, scope, this_ptr);
 
                 (custom_def.func)(&mut context, &expressions, &custom.state)
diff --git a/src/eval/global_state.rs b/src/eval/global_state.rs
index 98ba8263..6070ca11 100644
--- a/src/eval/global_state.rs
+++ b/src/eval/global_state.rs
@@ -111,8 +111,8 @@ impl GlobalRuntimeState {
             tag: engine.default_tag().clone(),
 
             #[cfg(feature = "debugging")]
-            debugger: engine.debugger.as_ref().map(|(init, ..)| {
-                crate::eval::Debugger::new(crate::eval::DebuggerStatus::Init, init(engine))
+            debugger: engine.debugger.as_ref().map(|x| {
+                crate::eval::Debugger::new(crate::eval::DebuggerStatus::Init, (x.0)(engine))
             }),
         }
     }
diff --git a/src/eval/stmt.rs b/src/eval/stmt.rs
index d0ddbfcc..198570a3 100644
--- a/src/eval/stmt.rs
+++ b/src/eval/stmt.rs
@@ -502,7 +502,8 @@ impl Engine {
                 #[cfg(not(feature = "no_module"))]
                 let func = func.or_else(|| global.get_iter(iter_type)).or_else(|| {
                     self.global_sub_modules
-                        .values()
+                        .iter()
+                        .flat_map(|m| m.values())
                         .find_map(|m| m.get_qualified_iter(iter_type))
                 });
 
diff --git a/src/func/call.rs b/src/func/call.rs
index 532c0892..591a5d4b 100644
--- a/src/func/call.rs
+++ b/src/func/call.rs
@@ -212,7 +212,8 @@ impl Engine {
                     } else {
                         func.or_else(|| _global.get_qualified_fn(hash)).or_else(|| {
                             self.global_sub_modules
-                                .values()
+                                .iter()
+                                .flat_map(|m| m.values())
                                 .find_map(|m| m.get_qualified_fn(hash).map(|f| (f, m.id_raw())))
                         })
                     };
@@ -252,7 +253,8 @@ impl Engine {
                             || _global.may_contain_dynamic_fn(hash_base)
                             || self
                                 .global_sub_modules
-                                .values()
+                                .iter()
+                                .flat_map(|m| m.values())
                                 .any(|m| m.may_contain_dynamic_fn(hash_base));
 
                         // Set maximum bitmask when there are dynamic versions of the function
diff --git a/src/func/script.rs b/src/func/script.rs
index efdc69de..b1352450 100644
--- a/src/func/script.rs
+++ b/src/func/script.rs
@@ -236,7 +236,7 @@ impl Engine {
             // Then check imported modules
             global.contains_qualified_fn(hash_script)
             // Then check sub-modules
-            || self.global_sub_modules.values().any(|m| m.contains_qualified_fn(hash_script));
+            || self.global_sub_modules.iter().flat_map(|m| m.values()).any(|m| m.contains_qualified_fn(hash_script));
 
         if !result && !cache.filter.is_absent_and_set(hash_script) {
             // Do not cache "one-hit wonders"
diff --git a/src/optimizer.rs b/src/optimizer.rs
index 63b519f8..4a3bc5ad 100644
--- a/src/optimizer.rs
+++ b/src/optimizer.rs
@@ -179,7 +179,8 @@ fn has_native_fn_override(
     #[cfg(not(feature = "no_module"))]
     if engine
         .global_sub_modules
-        .values()
+        .iter()
+        .flat_map(|m| m.values())
         .any(|m| m.contains_qualified_fn(hash))
     {
         return true;
diff --git a/src/packages/lang_core.rs b/src/packages/lang_core.rs
index 40d1f1b2..a4396514 100644
--- a/src/packages/lang_core.rs
+++ b/src/packages/lang_core.rs
@@ -267,7 +267,8 @@ fn collect_fn_metadata(
     #[cfg(not(feature = "no_module"))]
     ctx.engine()
         .global_sub_modules
-        .values()
+        .iter()
+        .flat_map(|m| m.values())
         .flat_map(|m| m.iter_script_fn())
         .filter(|(ns, a, n, p, f)| filter(*ns, *a, n, *p, f))
         .for_each(|(.., f)| {
diff --git a/src/parser.rs b/src/parser.rs
index d1882614..d299e121 100644
--- a/src/parser.rs
+++ b/src/parser.rs
@@ -600,7 +600,10 @@ impl Engine {
                         && index.is_none()
                         && !is_global
                         && !state.global_imports.iter().any(|m| m.as_str() == root)
-                        && !self.global_sub_modules.contains_key(root)
+                        && !self
+                            .global_sub_modules
+                            .as_ref()
+                            .map_or(false, |m| m.contains_key(root))
                     {
                         return Err(
                             PERR::ModuleUndefined(root.into()).into_err(namespace.position())
@@ -668,7 +671,10 @@ impl Engine {
                             && index.is_none()
                             && !is_global
                             && !state.global_imports.iter().any(|m| m.as_str() == root)
-                            && !self.global_sub_modules.contains_key(root)
+                            && !self
+                                .global_sub_modules
+                                .as_ref()
+                                .map_or(false, |m| m.contains_key(root))
                         {
                             return Err(
                                 PERR::ModuleUndefined(root.into()).into_err(namespace.position())
@@ -1550,9 +1556,16 @@ impl Engine {
             // Custom syntax.
             #[cfg(not(feature = "no_custom_syntax"))]
             Token::Custom(key) | Token::Reserved(key) | Token::Identifier(key)
-                if !self.custom_syntax.is_empty() && self.custom_syntax.contains_key(&**key) =>
+                if self
+                    .custom_syntax
+                    .as_ref()
+                    .map_or(false, |m| m.contains_key(&**key)) =>
             {
-                let (key, syntax) = self.custom_syntax.get_key_value(&**key).unwrap();
+                let (key, syntax) = self
+                    .custom_syntax
+                    .as_ref()
+                    .and_then(|m| m.get_key_value(&**key))
+                    .unwrap();
                 let (.., pos) = input.next().expect(NEVER_ENDS);
                 let settings2 = settings.level_up()?;
                 self.parse_custom_syntax(input, state, lib, settings2, key, syntax, pos)?
@@ -1856,7 +1869,10 @@ impl Engine {
                         && index.is_none()
                         && !is_global
                         && !state.global_imports.iter().any(|m| m.as_str() == root)
-                        && !self.global_sub_modules.contains_key(root)
+                        && !self
+                            .global_sub_modules
+                            .as_ref()
+                            .map_or(false, |m| m.contains_key(root))
                     {
                         return Err(
                             PERR::ModuleUndefined(root.into()).into_err(namespace.position())
@@ -2297,7 +2313,8 @@ impl Engine {
                 #[cfg(not(feature = "no_custom_syntax"))]
                 Token::Custom(c) => self
                     .custom_keywords
-                    .get(&**c)
+                    .as_ref()
+                    .and_then(|m| m.get(&**c))
                     .copied()
                     .ok_or_else(|| PERR::Reserved(c.to_string()).into_err(*current_pos))?,
                 Token::Reserved(c) if !is_valid_identifier(c) => {
@@ -2322,7 +2339,8 @@ impl Engine {
                 #[cfg(not(feature = "no_custom_syntax"))]
                 Token::Custom(c) => self
                     .custom_keywords
-                    .get(&**c)
+                    .as_ref()
+                    .and_then(|m| m.get(&**c))
                     .copied()
                     .ok_or_else(|| PERR::Reserved(c.to_string()).into_err(*next_pos))?,
                 Token::Reserved(c) if !is_valid_identifier(c) => {
@@ -2414,7 +2432,8 @@ impl Engine {
                 Token::Custom(s)
                     if self
                         .custom_keywords
-                        .get(s.as_str())
+                        .as_ref()
+                        .and_then(|m| m.get(s.as_str()))
                         .map_or(false, Option::is_some) =>
                 {
                     op_base.hashes = if is_valid_script_function {
diff --git a/src/serde/metadata.rs b/src/serde/metadata.rs
index 2481c028..50c76d86 100644
--- a/src/serde/metadata.rs
+++ b/src/serde/metadata.rs
@@ -170,7 +170,7 @@ pub fn gen_metadata_to_json(
     let mut global = ModuleMetadata::new();
 
     #[cfg(not(feature = "no_module"))]
-    for (name, m) in &engine.global_sub_modules {
+    for (name, m) in engine.global_sub_modules.iter().flat_map(|m| m.iter()) {
         global.modules.insert(name, m.as_ref().into());
     }
 
diff --git a/src/tokenizer.rs b/src/tokenizer.rs
index df776d9b..8d7ee4a4 100644
--- a/src/tokenizer.rs
+++ b/src/tokenizer.rs
@@ -2435,7 +2435,7 @@ impl<'a> Iterator for TokenIterator<'a> {
             Some((Token::Reserved(s), pos)) => (match
                 (s.as_str(),
                     #[cfg(not(feature = "no_custom_syntax"))]
-                    self.engine.custom_keywords.contains_key(&*s),
+                    self.engine.custom_keywords.as_ref().map_or(false, |m| m.contains_key(&*s)),
                     #[cfg(feature = "no_custom_syntax")]
                     false
                 )
@@ -2472,7 +2472,7 @@ impl<'a> Iterator for TokenIterator<'a> {
                 #[cfg(feature = "no_custom_syntax")]
                 (.., true) => unreachable!("no custom operators"),
                 // Reserved keyword that is not custom and disabled.
-                (token, false) if self.engine.disabled_symbols.contains(token) => {
+                (token, false) if self.engine.disabled_symbols.as_ref().map_or(false,|m| m.contains(token)) => {
                     let msg = format!("reserved {} '{token}' is disabled", if is_valid_identifier(token) { "keyword"} else {"symbol"});
                     Token::LexError(LERR::ImproperSymbol(s.to_string(), msg).into())
                 },
@@ -2481,13 +2481,13 @@ impl<'a> Iterator for TokenIterator<'a> {
             }, pos),
             // Custom keyword
             #[cfg(not(feature = "no_custom_syntax"))]
-            Some((Token::Identifier(s), pos)) if self.engine.custom_keywords.contains_key(&*s) => {
+            Some((Token::Identifier(s), pos)) if self.engine.custom_keywords.as_ref().map_or(false,|m| m.contains_key(&*s)) => {
                 (Token::Custom(s), pos)
             }
             // Custom keyword/symbol - must be disabled
             #[cfg(not(feature = "no_custom_syntax"))]
-            Some((token, pos)) if token.is_literal() && self.engine.custom_keywords.contains_key(token.literal_syntax()) => {
-                if self.engine.disabled_symbols.contains(token.literal_syntax()) {
+            Some((token, pos)) if token.is_literal() && self.engine.custom_keywords.as_ref().map_or(false,|m| m.contains_key(token.literal_syntax())) => {
+                if self.engine.disabled_symbols.as_ref().map_or(false,|m| m.contains(token.literal_syntax())) {
                     // Disabled standard keyword/symbol
                     (Token::Custom(Box::new(token.literal_syntax().into())), pos)
                 } else {
@@ -2496,7 +2496,7 @@ impl<'a> Iterator for TokenIterator<'a> {
                 }
             }
             // Disabled symbol
-            Some((token, pos)) if token.is_literal() && self.engine.disabled_symbols.contains(token.literal_syntax()) => {
+            Some((token, pos)) if token.is_literal() && self.engine.disabled_symbols.as_ref().map_or(false,|m| m.contains(token.literal_syntax())) => {
                 (Token::Reserved(Box::new(token.literal_syntax().into())), pos)
             }
             // Normal symbol