diff --git a/Assignments/04.rkt b/Assignments/04.rkt
index ccfafae..9b62dfc 100644
--- a/Assignments/04.rkt
+++ b/Assignments/04.rkt
@@ -130,7 +130,7 @@
        (type-case Type (typecheck fn env)
          [(arrowT arg-type result-type)
           (let ([actual-type (typecheck arg env)])
-            (if (equal? arg-type actual-type)
+            (if (subtype? actual-type arg-type)  ;; Use subtype? to check compatibility
                 result-type
                 (error 'typecheck "argument type")))]
          [else (error 'typecheck "not function")])]