diff --git a/src/cryptonote_core/tx_pool.cpp b/src/cryptonote_core/tx_pool.cpp
index 0b7e5c199..3a6a3833d 100644
--- a/src/cryptonote_core/tx_pool.cpp
+++ b/src/cryptonote_core/tx_pool.cpp
@@ -911,7 +911,7 @@ namespace cryptonote
     return true;
   }
   //---------------------------------------------------------------------------------
-  bool tx_memory_pool::check_for_key_images(const std::vector<crypto::key_image>& key_images, std::vector<bool> spent) const
+  bool tx_memory_pool::check_for_key_images(const std::vector<crypto::key_image>& key_images, std::vector<bool>& spent) const
   {
     CRITICAL_REGION_LOCAL(m_transactions_lock);
     CRITICAL_REGION_LOCAL1(m_blockchain);
diff --git a/src/cryptonote_core/tx_pool.h b/src/cryptonote_core/tx_pool.h
index 895014d17..99182cd0d 100644
--- a/src/cryptonote_core/tx_pool.h
+++ b/src/cryptonote_core/tx_pool.h
@@ -301,7 +301,7 @@ namespace cryptonote
      *
      * @return true
      */
-    bool check_for_key_images(const std::vector<crypto::key_image>& key_images, std::vector<bool> spent) const;
+    bool check_for_key_images(const std::vector<crypto::key_image>& key_images, std::vector<bool>& spent) const;
 
     /**
      * @brief get a specific transaction from the pool