diff --git a/src/engine_settings.rs b/src/engine_settings.rs
index 56e9104c..e7757636 100644
--- a/src/engine_settings.rs
+++ b/src/engine_settings.rs
@@ -1,6 +1,6 @@
 //! Configuration settings for [`Engine`].
 
-use crate::stdlib::{format, string::String};
+use crate::stdlib::{format, num::NonZeroU8, string::String};
 use crate::token::{is_valid_identifier, Token};
 use crate::Engine;
 
@@ -214,10 +214,12 @@ impl Engine {
         self.disabled_symbols.insert(symbol.into());
         self
     }
-    /// Register a custom operator into the language.
+    /// Register a custom operator with a precedence into the language.
     ///
     /// The operator must be a valid identifier (i.e. it cannot be a symbol).
     ///
+    /// The precedence cannot be zero.
+    ///
     /// # Example
     ///
     /// ```rust
@@ -245,6 +247,11 @@ impl Engine {
         keyword: &str,
         precedence: u8,
     ) -> Result<&mut Self, String> {
+        let precedence = NonZeroU8::new(precedence);
+
+        if precedence.is_none() {
+            return Err("precedence cannot be zero".into());
+        }
         if !is_valid_identifier(keyword.chars()) {
             return Err(format!("not a valid identifier: '{}'", keyword).into());
         }
@@ -259,8 +266,7 @@ impl Engine {
         }
 
         // Add to custom keywords
-        self.custom_keywords
-            .insert(keyword.into(), Some(precedence));
+        self.custom_keywords.insert(keyword.into(), precedence);
 
         Ok(self)
     }
diff --git a/src/syntax.rs b/src/syntax.rs
index 4ceec8f9..55db8cce 100644
--- a/src/syntax.rs
+++ b/src/syntax.rs
@@ -208,7 +208,7 @@ impl Engine {
     /// * `parse` is the parsing function.
     /// * `func` is the implementation function.
     ///
-    /// All custom keywords must be manually registered via [`register_custom_operator`][Engine::register_custom_operator].
+    /// All custom keywords must be manually registered via [`Engine::register_custom_operator`].
     /// Otherwise, custom keywords won't be recognized.
     pub fn register_custom_syntax_raw(
         &mut self,