diff --git a/class.c b/class.c
index 3c81132ce3..d743fb3fbb 100644
--- a/class.c
+++ b/class.c
@@ -278,10 +278,12 @@ class_duplicate_iclass_classext(VALUE iclass, rb_classext_t *mod_ext, const rb_n
     else {
         RCLASSEXT_M_TBL(ext) = RCLASSEXT_M_TBL(mod_ext);
     }
-    RCLASSEXT_FIELDS(ext) = (VALUE *)st_init_numtable();
+
     RCLASSEXT_CONST_TBL(ext) = RCLASSEXT_CONST_TBL(mod_ext);
     RCLASSEXT_CVC_TBL(ext) = RCLASSEXT_CVC_TBL(mod_ext);
 
+    RUBY_ASSERT(!RCLASSEXT_FIELDS(mod_ext));
+
     // Those are cache and should be recreated when methods are called
     // RCLASSEXT_CALLABLE_M_TBL(ext) = NULL;
     // RCLASSEXT_CC_TBL(ext) = NULL;