diff --git a/src/main.cpp b/src/main.cpp
index 1bcde7710..54683db07 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -367,7 +367,8 @@ void ProcessBlockAvailability(NodeId nodeid) {
 
     if (!state->hashLastUnknownBlock.IsNull()) {
         BlockMap::iterator itOld = mapBlockIndex.find(state->hashLastUnknownBlock);
-        if (itOld != mapBlockIndex.end() && itOld->second->nChainWork > 0) {
+        if (itOld != 0 && itOld->second != 0 && itOld != mapBlockIndex.end() && itOld->second->nChainWork > 0)
+        {
             if (state->pindexBestKnownBlock == NULL || itOld->second->nChainWork >= state->pindexBestKnownBlock->nChainWork)
                 state->pindexBestKnownBlock = itOld->second;
             state->hashLastUnknownBlock.SetNull();