diff --git a/src/chain.h b/src/chain.h
index c2a9cba1e6..fbc033281f 100644
--- a/src/chain.h
+++ b/src/chain.h
@@ -311,7 +311,9 @@ public:
 
     //! Check whether this block index entry is valid up to the passed validity level.
     bool IsValid(enum BlockStatus nUpTo = BLOCK_VALID_TRANSACTIONS) const
+        EXCLUSIVE_LOCKS_REQUIRED(::cs_main)
     {
+        AssertLockHeld(::cs_main);
         assert(!(nUpTo & ~BLOCK_VALID_MASK)); // Only validity flags allowed.
         if (nStatus & BLOCK_FAILED_MASK)
             return false;