diff --git a/gc.c b/gc.c
index 5240852cc7..05d75e16e2 100644
--- a/gc.c
+++ b/gc.c
@@ -6328,7 +6328,7 @@ gc_verify_internal_consistency(rb_objspace_t *objspace)
                    objspace->rgengc.old_objects, data.old_object_count);
 	}
 	if (objspace->rgengc.uncollectible_wb_unprotected_objects != data.remembered_shady_count) {
-	    rb_bug("inconsistent old slot number: expect %"PRIuSIZE", but %"PRIuSIZE".",
+            rb_bug("inconsistent number of wb unprotected objects: expect %"PRIuSIZE", but %"PRIuSIZE".",
                    objspace->rgengc.uncollectible_wb_unprotected_objects, data.remembered_shady_count);
 	}
     }