diff --git a/vm/alloc/MarkSweep.c b/vm/alloc/MarkSweep.c
index 1fb3f24..8bb2de6 100644
--- a/vm/alloc/MarkSweep.c
+++ b/vm/alloc/MarkSweep.c
@@ -478,6 +478,7 @@
 {
     assert(obj != NULL);
     assert(ctx != NULL);
+    assert(isMarked(obj, ctx));
     assert(obj->clazz != NULL);
     if (obj->clazz == gDvm.classJavaLangClass) {
         scanClassObject(obj, ctx);