diff --git a/src/main.cpp b/src/main.cpp
index 119967a46..fa402338a 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -5147,7 +5147,8 @@ void static CheckBlockIndex()
     // Build forward-pointing map of the entire block tree.
     std::multimap<CBlockIndex*,CBlockIndex*> forward;
     for (BlockMap::iterator it = mapBlockIndex.begin(); it != mapBlockIndex.end(); it++) {
-        forward.insert(std::make_pair(it->second->pprev, it->second));
+        if ( it->second != 0 )
+            forward.insert(std::make_pair(it->second->pprev, it->second));
     }
     
     assert(forward.size() == mapBlockIndex.size());