diff --git a/src/main.cpp b/src/main.cpp
index 0bbe833705..a591168c07 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -1727,6 +1727,7 @@ void ThreadScriptCheck() {
 
 bool ConnectBlock(CBlock& block, CValidationState& state, CBlockIndex* pindex, CCoinsViewCache& view, bool fJustCheck)
 {
+    AssertLockHeld(cs_main);
     // Check it again in case a previous version let a bad block in
     if (!CheckBlock(block, state, !fJustCheck, !fJustCheck))
         return false;