diff --git a/src/txmempool.cpp b/src/txmempool.cpp
index d1edde284f7..cc639288d37 100644
--- a/src/txmempool.cpp
+++ b/src/txmempool.cpp
@@ -339,7 +339,7 @@ CTxMemPool::CTxMemPool(CBlockPolicyEstimator* estimator) :
     nCheckFrequency = 0;
 }
 
-bool CTxMemPool::isSpent(const COutPoint& outpoint)
+bool CTxMemPool::isSpent(const COutPoint& outpoint) const
 {
     LOCK(cs);
     return mapNextTx.count(outpoint);
diff --git a/src/txmempool.h b/src/txmempool.h
index 08a34210159..699f6b554bd 100644
--- a/src/txmempool.h
+++ b/src/txmempool.h
@@ -550,7 +550,7 @@ public:
     void _clear(); //lock free
     bool CompareDepthAndScore(const uint256& hasha, const uint256& hashb);
     void queryHashes(std::vector<uint256>& vtxid);
-    bool isSpent(const COutPoint& outpoint);
+    bool isSpent(const COutPoint& outpoint) const;
     unsigned int GetTransactionsUpdated() const;
     void AddTransactionsUpdated(unsigned int n);
     /**