diff --git a/vm.c b/vm.c
index 7c4d6fe2c1..22ade689fc 100644
--- a/vm.c
+++ b/vm.c
@@ -1142,7 +1142,7 @@ rb_vm_cref(void)
     rb_thread_t *th = GET_THREAD();
     rb_control_frame_t *cfp = rb_vm_get_ruby_level_next_cfp(th, th->cfp);
 
-    if (cfp == 0) {
+    if (cfp == NULL) {
 	return NULL;
     }