diff --git a/README.md b/README.md
index 6edb1ca7..f6db9187 100644
--- a/README.md
+++ b/README.md
@@ -2347,7 +2347,7 @@ engine.set_max_modules(0);                  // allow unlimited modules
 
 ### Maximum call stack depth
 
-Rhai by default limits function calls to a maximum depth of 128 levels (8 levels in debug build).
+Rhai by default limits function calls to a maximum depth of 128 levels (16 levels in debug build).
 This limit may be changed via the `Engine::set_max_call_levels` method.
 
 When setting this limit, care must be also taken to the evaluation depth of each _statement_
diff --git a/src/engine.rs b/src/engine.rs
index 6bd2bafa..bd28ccf1 100644
--- a/src/engine.rs
+++ b/src/engine.rs
@@ -27,7 +27,7 @@ use crate::stdlib::{
     format,
     iter::{empty, once, repeat},
     mem,
-    num::{NonZeroU64, NonZeroUsize},
+    num::NonZeroUsize,
     ops::{Deref, DerefMut},
     rc::Rc,
     string::{String, ToString},
@@ -49,7 +49,7 @@ pub type Map = HashMap<String, Dynamic>;
 
 #[cfg(not(feature = "unchecked"))]
 #[cfg(debug_assertions)]
-pub const MAX_CALL_STACK_DEPTH: usize = 8;
+pub const MAX_CALL_STACK_DEPTH: usize = 16;
 #[cfg(not(feature = "unchecked"))]
 #[cfg(debug_assertions)]
 pub const MAX_EXPR_DEPTH: usize = 32;
@@ -354,16 +354,16 @@ pub struct Engine {
     pub(crate) optimization_level: OptimizationLevel,
     /// Maximum levels of call-stack to prevent infinite recursion.
     ///
-    /// Defaults to 8 for debug builds and 128 for non-debug builds.
+    /// Defaults to 16 for debug builds and 128 for non-debug builds.
     pub(crate) max_call_stack_depth: usize,
     /// Maximum depth of statements/expressions at global level.
     pub(crate) max_expr_depth: usize,
     /// Maximum depth of statements/expressions in functions.
     pub(crate) max_function_expr_depth: usize,
     /// Maximum number of operations allowed to run.
-    pub(crate) max_operations: Option<NonZeroU64>,
+    pub(crate) max_operations: u64,
     /// Maximum number of modules allowed to load.
-    pub(crate) max_modules: Option<NonZeroU64>,
+    pub(crate) max_modules: u64,
 }
 
 impl Default for Engine {
@@ -404,8 +404,8 @@ impl Default for Engine {
             max_call_stack_depth: MAX_CALL_STACK_DEPTH,
             max_expr_depth: MAX_EXPR_DEPTH,
             max_function_expr_depth: MAX_FUNCTION_EXPR_DEPTH,
-            max_operations: None,
-            max_modules: None,
+            max_operations: u64::MAX,
+            max_modules: u64::MAX,
         };
 
         #[cfg(feature = "no_stdlib")]
@@ -547,8 +547,8 @@ impl Engine {
             max_call_stack_depth: MAX_CALL_STACK_DEPTH,
             max_expr_depth: MAX_EXPR_DEPTH,
             max_function_expr_depth: MAX_FUNCTION_EXPR_DEPTH,
-            max_operations: None,
-            max_modules: None,
+            max_operations: u64::MAX,
+            max_modules: u64::MAX,
         }
     }
 
@@ -589,13 +589,17 @@ impl Engine {
     /// consuming too much resources (0 for unlimited).
     #[cfg(not(feature = "unchecked"))]
     pub fn set_max_operations(&mut self, operations: u64) {
-        self.max_operations = NonZeroU64::new(operations);
+        self.max_operations = if operations == 0 {
+            u64::MAX
+        } else {
+            operations
+        };
     }
 
     /// Set the maximum number of imported modules allowed for a script (0 for unlimited).
     #[cfg(not(feature = "unchecked"))]
     pub fn set_max_modules(&mut self, modules: u64) {
-        self.max_modules = NonZeroU64::new(modules);
+        self.max_modules = if modules == 0 { u64::MAX } else { modules };
     }
 
     /// Set the depth limits for expressions/statements.
@@ -1835,10 +1839,8 @@ impl Engine {
                     let (expr, (name, pos)) = x.as_ref();
 
                     // Guard against too many modules
-                    if let Some(max) = self.max_modules {
-                        if state.modules >= max.get() {
-                            return Err(Box::new(EvalAltResult::ErrorTooManyModules(*pos)));
-                        }
+                    if state.modules >= self.max_modules {
+                        return Err(Box::new(EvalAltResult::ErrorTooManyModules(*pos)));
                     }
 
                     if let Some(path) = self
@@ -1902,10 +1904,8 @@ impl Engine {
         #[cfg(not(feature = "unchecked"))]
         {
             // Guard against too many operations
-            if let Some(max) = self.max_operations {
-                if state.operations > max.get() {
-                    return Err(Box::new(EvalAltResult::ErrorTooManyOperations(pos)));
-                }
+            if state.operations > self.max_operations {
+                return Err(Box::new(EvalAltResult::ErrorTooManyOperations(pos)));
             }
         }
 
diff --git a/tests/modules.rs b/tests/modules.rs
index 9c6c8ec4..4e475d1c 100644
--- a/tests/modules.rs
+++ b/tests/modules.rs
@@ -86,14 +86,14 @@ fn test_module_resolver() -> Result<(), Box<EvalAltResult>> {
         *engine
             .eval::<INT>(
                 r#"
-                    let x = 0;
+                    let sum = 0;
 
                     for x in range(0, 10) {
                         import "hello" as h;
-                        x += h::answer;
+                        sum += h::answer;
                     }
 
-                    x
+                    sum
             "#
             )
             .expect_err("should error"),
@@ -105,18 +105,18 @@ fn test_module_resolver() -> Result<(), Box<EvalAltResult>> {
         *engine
             .eval::<INT>(
                 r#"
-                    let x = 0;
+                    let sum = 0;
 
                     fn foo() {
                         import "hello" as h;
-                        x += h::answer;
+                        sum += h::answer;
                     }
 
                     for x in range(0, 10) {
                         foo();
                     }
 
-                    x
+                    sum
             "#
             )
             .expect_err("should error"),
diff --git a/tests/stack.rs b/tests/stack.rs
index 3087e558..ee7eedc4 100644
--- a/tests/stack.rs
+++ b/tests/stack.rs
@@ -16,19 +16,14 @@ fn test_stack_overflow_fn_calls() -> Result<(), Box<EvalAltResult>> {
     );
 
     #[cfg(not(feature = "unchecked"))]
-    match engine.eval::<()>(
+    assert!(matches!(
+        *engine.eval::<()>(
         r"
             fn foo(n) { if n == 0 { 0 } else { n + foo(n-1) } }
             foo(1000)
-    ",
-    ) {
-        Ok(_) => panic!("should be stack overflow"),
-        Err(err) => match *err {
-            EvalAltResult::ErrorInFunctionCall(name, _, _)
-                if name.starts_with("foo > foo > foo") => {}
-            _ => panic!("should be stack overflow"),
-        },
-    }
+    ").expect_err("should error"),
+        EvalAltResult::ErrorInFunctionCall(name, _, _) if name.starts_with("foo > foo > foo")
+    ));
 
     Ok(())
 }