diff --git a/gc.c b/gc.c
index 84e9b20d4f..099a1f382b 100644
--- a/gc.c
+++ b/gc.c
@@ -9560,19 +9560,21 @@ gc_move(rb_objspace_t *objspace, VALUE scan, VALUE free, size_t src_slot_size, s
     }
 
     if (FL_TEST((VALUE)src, FL_SEEN_OBJ_ID)) {
-        st_data_t srcid = (st_data_t)src, id;
         /* If the source object's object_id has been seen, we need to update
          * the object to object id mapping. */
-        if (st_lookup(objspace->obj_to_id_tbl, srcid, &id)) {
-            gc_report(4, objspace, "Moving object with seen id: %p -> %p\n", (void *)src, (void *)dest);
-            /* Resizing the st table could cause a malloc */
-            DURING_GC_COULD_MALLOC_REGION_START();
-            {
-                st_delete(objspace->obj_to_id_tbl, &srcid, 0);
-                st_insert(objspace->obj_to_id_tbl, (st_data_t)dest, id);
+        st_data_t srcid = (st_data_t)src, id;
+
+        gc_report(4, objspace, "Moving object with seen id: %p -> %p\n", (void *)src, (void *)dest);
+        /* Resizing the st table could cause a malloc */
+        DURING_GC_COULD_MALLOC_REGION_START();
+        {
+            if (!st_delete(objspace->obj_to_id_tbl, &srcid, &id)) {
+                rb_bug("gc_move: object ID seen, but not in mapping table: %s", obj_info((VALUE)src));
             }
-            DURING_GC_COULD_MALLOC_REGION_END();
+
+            st_insert(objspace->obj_to_id_tbl, (st_data_t)dest, id);
         }
+        DURING_GC_COULD_MALLOC_REGION_END();
     }
     else {
         GC_ASSERT(!st_lookup(objspace->obj_to_id_tbl, (st_data_t)src, NULL));