diff --git a/gc.c b/gc.c
index 11a79e69fe..f1232d3aec 100644
--- a/gc.c
+++ b/gc.c
@@ -2597,6 +2597,10 @@ make_zombie(rb_objspace_t *objspace, VALUE obj, void (*dfree)(void *), void *dat
     zombie->data = data;
     zombie->next = heap_pages_deferred_final;
     heap_pages_deferred_final = (VALUE)zombie;
+
+    struct heap_page *page = GET_HEAP_PAGE(obj);
+    page->final_slots++;
+    heap_pages_final_slots++;
 }
 
 static inline void
@@ -3484,7 +3488,9 @@ finalize_list(rb_objspace_t *objspace, VALUE zombie)
         }
 
 	RZOMBIE(zombie)->basic.flags = 0;
-        if (LIKELY(heap_pages_final_slots)) heap_pages_final_slots--;
+        GC_ASSERT(heap_pages_final_slots > 0);
+        GC_ASSERT(page->final_slots > 0);
+        heap_pages_final_slots--;
 	page->final_slots--;
 	page->free_slots++;
 	heap_page_add_freeobj(objspace, GET_HEAP_PAGE(zombie), zombie);
@@ -4324,8 +4330,6 @@ gc_page_sweep(rb_objspace_t *objspace, rb_heap_t *heap, struct heap_page *sweep_
 
     sweep_page->free_slots = freed_slots + empty_slots;
     objspace->profile.total_freed_objects += freed_slots;
-    heap_pages_final_slots += final_slots;
-    sweep_page->final_slots += final_slots;
 
     if (heap_pages_deferred_final && !finalizing) {
         rb_thread_t *th = GET_THREAD();