diff --git a/exercises/traits/traits4.rs b/exercises/traits/traits4.rs
index 6b541665..23ebd239 100644
--- a/exercises/traits/traits4.rs
+++ b/exercises/traits/traits4.rs
@@ -20,7 +20,7 @@ impl Licensed for SomeSoftware {}
 impl Licensed for OtherSoftware {}
 
 // YOU MAY ONLY CHANGE THE NEXT LINE
-fn compare_license_types(software: ??, software_two: ??) -> bool {
+fn compare_license_types(software: impl Licensed, software_two: impl Licensed) -> bool {
     software.licensing_info() == software_two.licensing_info()
 }