diff --git a/src/rpcmining.cpp b/src/rpcmining.cpp
index 1e6531f68..949fe3ed5 100644
--- a/src/rpcmining.cpp
+++ b/src/rpcmining.cpp
@@ -629,14 +629,19 @@ Value submitblock(const Array& params, bool fHelp)
         throw JSONRPCError(RPC_DESERIALIZATION_ERROR, "Block decode failed");
 
     uint256 hash = block.GetHash();
-    BlockMap::iterator mi = mapBlockIndex.find(hash);
-    if (mi != mapBlockIndex.end()) {
-        CBlockIndex *pindex = mi->second;
-        if (pindex->IsValid(BLOCK_VALID_SCRIPTS))
-            return "duplicate";
-        if (pindex->nStatus & BLOCK_FAILED_MASK)
-            return "duplicate-invalid";
-        // Otherwise, we might only have the header - process the block before returning
+    bool fBlockPresent = false;
+    {
+        LOCK(cs_main);
+        BlockMap::iterator mi = mapBlockIndex.find(hash);
+        if (mi != mapBlockIndex.end()) {
+            CBlockIndex *pindex = mi->second;
+            if (pindex->IsValid(BLOCK_VALID_SCRIPTS))
+                return "duplicate";
+            if (pindex->nStatus & BLOCK_FAILED_MASK)
+                return "duplicate-invalid";
+            // Otherwise, we might only have the header - process the block before returning
+            fBlockPresent = true;
+        }
     }
 
     CValidationState state;
@@ -644,7 +649,7 @@ Value submitblock(const Array& params, bool fHelp)
     RegisterValidationInterface(&sc);
     bool fAccepted = ProcessNewBlock(state, NULL, &block);
     UnregisterValidationInterface(&sc);
-    if (mi != mapBlockIndex.end())
+    if (fBlockPresent)
     {
         if (fAccepted && !sc.found)
             return "duplicate-inconclusive";