diff --git a/src/main.cpp b/src/main.cpp
index 777926200..7fa68be9e 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -384,7 +384,7 @@ void UpdateBlockAvailability(NodeId nodeid, const uint256 &hash) {
     ProcessBlockAvailability(nodeid);
 
     BlockMap::iterator it = mapBlockIndex.find(hash);
-    if (it != mapBlockIndex.end() && it->second->nChainWork > 0) {
+    if (it->second != 0 && it != mapBlockIndex.end() && it->second->nChainWork > 0) {
         // An actually better block was announced.
         if (state->pindexBestKnownBlock == NULL || it->second->nChainWork >= state->pindexBestKnownBlock->nChainWork)
             state->pindexBestKnownBlock = it->second;