diff --git a/src/engine.rs b/src/engine.rs
index 9be3c04c..4ad31b36 100644
--- a/src/engine.rs
+++ b/src/engine.rs
@@ -708,7 +708,11 @@ impl Engine {
                 "{} ({})",
                 fn_name,
                 args.iter()
-                    .map(|name| self.map_type_name(name.type_name()))
+                    .map(|name| if name.is::<ImmutableString>() {
+                        "&str | ImmutableString"
+                    } else {
+                        self.map_type_name(name.type_name())
+                    })
                     .collect::<Vec<_>>()
                     .join(", ")
             ),
diff --git a/tests/decrement.rs b/tests/decrement.rs
index ab1f9376..1b74dfe9 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 == "- (string, string)"
+        EvalAltResult::ErrorFunctionNotFound(err, _) if err == "- (&str | ImmutableString, &str | ImmutableString)"
     ));
 
     Ok(())
diff --git a/tests/string.rs b/tests/string.rs
index 27854629..6eed9e6a 100644
--- a/tests/string.rs
+++ b/tests/string.rs
@@ -168,7 +168,7 @@ fn test_string_fn() -> Result<(), Box<EvalAltResult>> {
 
     assert!(matches!(
         *engine.eval::<INT>(r#"foo3("hello")"#).expect_err("should error"),
-        EvalAltResult::ErrorFunctionNotFound(ref x, _) if x == "foo3 (string)"
+        EvalAltResult::ErrorFunctionNotFound(err, _) if err == "foo3 (&str | ImmutableString)"
     ));
 
     Ok(())