diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6523076d..85615fb3 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -10,6 +10,7 @@ Bug fixes
 * Panic when passing a shared string into a registered function as `&str` argument is fixed.
 * Panic when calling `switch` statements on custom types is fixed.
 * Potential overflow panics in `range(from, to, step)` is fixed.
+* `&mut String` parameters in registered functions no longer panic when passed a string.
 * Some expressions involving shared variables now work properly, for example `x in shared_value`, `return shared_value`, `obj.field = shared_value` etc. Previously, the resultant value is still shared which is counter-intuitive.
 * Errors in native Rust functions now contain the correct function call positions.
 * Fixed error types in `EvalAltResult::ErrorMismatchDataType` which were swapped.
diff --git a/src/fn_register.rs b/src/fn_register.rs
index 74ca32be..5751f0d3 100644
--- a/src/fn_register.rs
+++ b/src/fn_register.rs
@@ -161,15 +161,15 @@ pub fn map_result(data: RhaiResult) -> RhaiResult {
 
 /// Remap `&str` | `String` to `ImmutableString`.
 #[inline(always)]
-fn map_type_id<T: 'static>() -> TypeId {
-    let id = TypeId::of::<T>();
+fn map_type_id<R: 'static, T: 'static>() -> TypeId {
+    let ref_id = TypeId::of::<R>();
 
-    if id == TypeId::of::<&str>() {
+    if ref_id == TypeId::of::<&str>() {
         TypeId::of::<ImmutableString>()
-    } else if id == TypeId::of::<String>() {
+    } else if ref_id == TypeId::of::<String>() {
         TypeId::of::<ImmutableString>()
     } else {
-        id
+        TypeId::of::<T>()
     }
 }
 
@@ -193,7 +193,7 @@ macro_rules! def_register {
             #[inline(always)]
             fn register_fn(&mut self, name: &str, f: FN) -> &mut Self {
                 self.global_namespace.set_fn(name, FnNamespace::Global, FnAccess::Public, None,
-                    &[$(map_type_id::<$par>()),*],
+                    &[$(map_type_id::<$param, $par>()),*],
                     CallableFunction::$abi(make_func!(f : map_dynamic ; $($par => $let => $clone => $arg),*))
                 );
                 self
@@ -208,7 +208,7 @@ macro_rules! def_register {
             #[inline(always)]
             fn register_result_fn(&mut self, name: &str, f: FN) -> &mut Self {
                 self.global_namespace.set_fn(name, FnNamespace::Global, FnAccess::Public, None,
-                    &[$(map_type_id::<$par>()),*],
+                    &[$(map_type_id::<$param, $par>()),*],
                     CallableFunction::$abi(make_func!(f : map_result ; $($par => $let => $clone => $arg),*))
                 );
                 self
diff --git a/tests/string.rs b/tests/string.rs
index a5328fad..0db10eb0 100644
--- a/tests/string.rs
+++ b/tests/string.rs
@@ -64,6 +64,25 @@ fn test_string_dynamic() -> Result<(), Box<EvalAltResult>> {
     Ok(())
 }
 
+#[test]
+fn test_string_mut() -> Result<(), Box<EvalAltResult>> {
+    let mut engine = Engine::new();
+
+    engine.register_fn("foo", |x: INT, s: &str| s.len() as INT + x);
+    engine.register_fn("bar", |x: INT, s: String| s.len() as INT + x);
+    engine.register_fn("baz", |s: &mut String| s.len());
+
+    assert_eq!(engine.eval::<INT>(r#"foo(1, "hello")"#)?, 6);
+    assert_eq!(engine.eval::<INT>(r#"bar(1, "hello")"#)?, 6);
+    assert!(
+        matches!(*engine.eval::<INT>(r#"baz("hello")"#).expect_err("should error"),
+            EvalAltResult::ErrorFunctionNotFound(f, _) if f == "baz (&str | ImmutableString | String)"
+        )
+    );
+
+    Ok(())
+}
+
 #[cfg(not(feature = "no_object"))]
 #[test]
 fn test_string_substring() -> Result<(), Box<EvalAltResult>> {