diff --git a/src/engine.rs b/src/engine.rs
index 0455c6f1..1c39120f 100644
--- a/src/engine.rs
+++ b/src/engine.rs
@@ -855,7 +855,7 @@ impl Engine {
                 fn_name,
                 args.iter()
                     .map(|name| if name.is::<ImmutableString>() {
-                        "&str | ImmutableString"
+                        "&str | ImmutableString | String"
                     } else {
                         self.map_type_name((*name).type_name())
                     })
diff --git a/src/fn_register.rs b/src/fn_register.rs
index 7cf7be74..23d8043f 100644
--- a/src/fn_register.rs
+++ b/src/fn_register.rs
@@ -6,6 +6,7 @@ use crate::engine::Engine;
 use crate::fn_native::{CallableFunction, FnAny, FnCallArgs, SendSync};
 use crate::module::Module;
 use crate::parser::FnAccess;
+use crate::r#unsafe::unsafe_cast_box;
 use crate::result::EvalAltResult;
 use crate::utils::ImmutableString;
 
@@ -105,6 +106,9 @@ pub fn by_value<T: Variant + Clone>(data: &mut Dynamic) -> T {
         let ref_str = data.as_str().unwrap();
         let ref_T = unsafe { mem::transmute::<_, &T>(&ref_str) };
         ref_T.clone()
+    } else if TypeId::of::<T>() == TypeId::of::<String>() {
+        // If T is String, data must be ImmutableString, so map directly to it
+        *unsafe_cast_box(Box::new(data.as_str().unwrap().to_string())).unwrap()
     } else {
         // We consume the argument and then replace it with () - the argument is not supposed to be used again.
         // This way, we avoid having to clone the argument again, because it is already a clone when passed here.
@@ -154,13 +158,15 @@ pub fn map_result(
     data
 }
 
-/// Remap `&str` to `ImmutableString`.
+/// Remap `&str` | `String` to `ImmutableString`.
 #[inline(always)]
 fn map_type_id<T: 'static>() -> TypeId {
     let id = TypeId::of::<T>();
 
     if id == TypeId::of::<&str>() {
         TypeId::of::<ImmutableString>()
+    } else if id == TypeId::of::<String>() {
+        TypeId::of::<ImmutableString>()
     } else {
         id
     }
diff --git a/tests/decrement.rs b/tests/decrement.rs
index 1b74dfe9..0691eddf 100644
--- a/tests/decrement.rs
+++ b/tests/decrement.rs
@@ -8,7 +8,7 @@ fn test_decrement() -> Result<(), Box<EvalAltResult>> {
 
     assert!(matches!(
         *engine.eval::<String>(r#"let s = "test"; s -= "ing"; s"#).expect_err("expects error"),
-        EvalAltResult::ErrorFunctionNotFound(err, _) if err == "- (&str | ImmutableString, &str | ImmutableString)"
+        EvalAltResult::ErrorFunctionNotFound(err, _) if err == "- (&str | ImmutableString | String, &str | ImmutableString | String)"
     ));
 
     Ok(())
diff --git a/tests/string.rs b/tests/string.rs
index e6b77841..908b4c6c 100644
--- a/tests/string.rs
+++ b/tests/string.rs
@@ -173,17 +173,16 @@ fn test_string_fn() -> Result<(), Box<EvalAltResult>> {
         "foo"
     );
 
-    engine.register_fn("foo1", |s: &str| s.len() as INT);
-    engine.register_fn("foo2", |s: ImmutableString| s.len() as INT);
-    engine.register_fn("foo3", |s: String| s.len() as INT);
+    engine
+        .register_fn("foo1", |s: &str| s.len() as INT)
+        .register_fn("foo2", |s: ImmutableString| s.len() as INT)
+        .register_fn("foo3", |s: String| s.len() as INT)
+        .register_fn("foo4", |s: &mut ImmutableString| s.len() as INT);
 
     assert_eq!(engine.eval::<INT>(r#"foo1("hello")"#)?, 5);
     assert_eq!(engine.eval::<INT>(r#"foo2("hello")"#)?, 5);
-
-    assert!(matches!(
-        *engine.eval::<INT>(r#"foo3("hello")"#).expect_err("should error"),
-        EvalAltResult::ErrorFunctionNotFound(err, _) if err == "foo3 (&str | ImmutableString)"
-    ));
+    assert_eq!(engine.eval::<INT>(r#"foo3("hello")"#)?, 5);
+    assert_eq!(engine.eval::<INT>(r#"foo4("hello")"#)?, 5);
 
     Ok(())
 }