diff --git a/compile.c b/compile.c
index 7de791a033..bd691627e3 100644
--- a/compile.c
+++ b/compile.c
@@ -1290,10 +1290,15 @@ static void
 APPEND_LIST(ISEQ_ARG_DECLARE LINK_ANCHOR *const anc1, LINK_ANCHOR *const anc2)
 {
     if (anc2->anchor.next) {
+        /* LINK_ANCHOR must not loop */
+        RUBY_ASSERT(anc2->last != &anc2->anchor);
         anc1->last->next = anc2->anchor.next;
         anc2->anchor.next->prev = anc1->last;
         anc1->last = anc2->last;
     }
+    else {
+        RUBY_ASSERT(anc2->last == &anc2->anchor);
+    }
     verify_list("append", anc1);
 }
 #if CPDEBUG < 0