diff --git a/eqpalg/utility/expression_engine.h b/eqpalg/utility/expression_engine.h
index 2b3edc0..a1af07c 100644
--- a/eqpalg/utility/expression_engine.h
+++ b/eqpalg/utility/expression_engine.h
@@ -119,7 +119,9 @@ private:
   static constexpr size_t PV_NUM = 6;
 
   static void ensureVarCache(VarsCache& vc, size_t tag_count) {
-    if (vc.tag_num == 0 && tag_count > 0) {
+    // VarsCache::tag_num 是未初始化的垃圾值，不能用来判断是否已初始化
+    // 直接 init()，内部 resize 操作对已初始化的向量是安全的
+    if (tag_count > 0 && vc.tag_keys.size() != tag_count) {
       vc.init(tag_count, 6);
     }
   }