diff --git a/src/index/base.cpp b/src/index/base.cpp
--- a/src/index/base.cpp
+++ b/src/index/base.cpp
@@ -66,7 +66,8 @@
     return true;
 }
 
-static const CBlockIndex *NextSyncBlock(const CBlockIndex *pindex_prev) {
+static const CBlockIndex *NextSyncBlock(const CBlockIndex *pindex_prev)
+    EXCLUSIVE_LOCKS_REQUIRED(cs_main) {
     AssertLockHeld(cs_main);
 
     if (!pindex_prev) {