diff --git a/src/main.cpp b/src/main.cpp
index 4e1c20de2a..9c0f25950a 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -5442,6 +5442,8 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
         BlockTransactionsRequest req;
         vRecv >> req;
 
+        LOCK(cs_main);
+
         BlockMap::iterator it = mapBlockIndex.find(req.blockhash);
         if (it == mapBlockIndex.end() || !(it->second->nStatus & BLOCK_HAVE_DATA)) {
             LogPrintf("Peer %d sent us a getblocktxn for a block we don't have", pfrom->id);