--- x/kernel/rcu/tree.c
+++ y/kernel/rcu/tree.c
@@ -3071,6 +3071,7 @@ __call_rcu_common(struct rcu_head *head,
 
 	/* Misaligned rcu_head! */
 	WARN_ON_ONCE((unsigned long)head & (sizeof(void *) - 1));
+	BUG_ON(!func);
 
 	if (debug_rcu_head_queue(head)) {
 		/*