diff --git a/src/main.cpp b/src/main.cpp
index 33bd98928..2b6966659 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -4374,6 +4374,7 @@ bool CheckBlock(int32_t *futureblockp,int32_t height,CBlockIndex *pindex,const C
         // here we add back all txs from the temp mempool to the main mempool.
         // which removes any tx locally that were invalid after the block arrives.
         int invalidtxs = 0;
+        LOCK(mempool.cs);
         BOOST_FOREACH(const CTxMemPoolEntry& e, tmpmempool.mapTx) {
             CTransaction tx = e.GetTx();
             CValidationState state; bool fMissingInputs,fOverrideFees = false;