diff --git a/speechx/speechx/utils/math.cc b/speechx/speechx/utils/math.cc
index 289470f6c6efaf8516f4bc00b60376a02761a760..959740a05a3fcaf879fab198a22ae2fee6777a1a 100644
--- a/speechx/speechx/utils/math.cc
+++ b/speechx/speechx/utils/math.cc
@@ -79,10 +79,12 @@ void TopK(const std::vector<T>& data,
     int cur = values->size() - 1;
     while (!pq.empty()) {
         const auto& item = pq.top();
-        pq.pop();
-
+    
         (*values)[cur] = item.first;
         (*indices)[cur] = item.second;
+         
+        // item if reference, must pop here
+        pq.pop();
 
         cur--;
     }
@@ -93,4 +95,4 @@ template void TopK<float>(const std::vector<float>& data,
                           std::vector<float>* values,
                           std::vector<int>* indices);
 
-}  // namespace ppspeech
\ No newline at end of file
+}  // namespace ppspeech