It was too early to check key for undef; ref #4534

src/hash.c

@@ -543,10 +543,10 @@ ht_copy(mrb_state *mrb, htable *t)
       mrb_value key = seg->e[i].key;
       mrb_value val = seg->e[i].val;
 
-      if (mrb_undef_p(key)) continue; /* skip deleted key */
       if ((seg->next == NULL) && (i >= t->last_len)) {
         return t2;
       }
+      if (mrb_undef_p(key)) continue; /* skip deleted key */
       ht_put(mrb, t2, key, val);
     }
     seg = seg->next;