diff --git a/Assignments/04.rkt b/Assignments/04.rkt
index 9b62dfc..286a887 100644
--- a/Assignments/04.rkt
+++ b/Assignments/04.rkt
@@ -83,14 +83,12 @@
                   (if (empty? keys)
                       #t
                       (let ([key (first keys)])
-                        (type-case (Optionof Type) (hash-ref Y-fields key)
-                          [(none) #f]  ;; Key not found in Y-fields
-                          [(some Y-type)
+                        (let ([Y-type (some-v (hash-ref Y-fields key))])
                            (type-case (Optionof Type) (hash-ref X-fields key)
                              [(none) #f]  ;; Key not found in X-fields
                              [(some X-type)
                               (and (subtype? X-type Y-type)  ;; Check subtyping of field types
-                                   (loop (rest keys)))])]))))]  ;; Recurse on remaining keys
+                                   (loop (rest keys)))])))))]  ;; Recurse on remaining keys
           (loop (hash-keys Y-fields)))]
        [else #f])]))
 
@@ -322,5 +320,11 @@
   (test (subtype? (arrowT hello-goodbye-t hello-goodbye-t)
                   (arrowT hello-t hello-t)) #f)
   ;; for coverage
+  (define non-object-fun `{lam {x : num} {msg x hello}})
+  ;; `x` is bound to `numT`, which is not an object type
+  (test/exn (tc non-object-fun) "bound variable is not an object")
+  (test (subtype? hello-t (boolT)) #f)
+  (test (subtype? (boolT) (boolT)) #t)
+  (test (subtype? (boolT) (numT)) #f)
   (test (subtype? (arrowT (numT) (numT)) (numT)) #f)
   (test (subtype? (numT) (arrowT (numT) (numT))) #f))
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