diff --git a/src/node/blockstorage.cpp b/src/node/blockstorage.cpp
index 9c3475e0e7..120b86d23e 100644
--- a/src/node/blockstorage.cpp
+++ b/src/node/blockstorage.cpp
@@ -331,6 +331,7 @@ void BlockManager::Unload()
 
 bool BlockManager::WriteBlockIndexDB()
 {
+    AssertLockHeld(::cs_main);
     std::vector<std::pair<int, const CBlockFileInfo*>> vFiles;
     vFiles.reserve(m_dirty_fileinfo.size());
     for (std::set<int>::iterator it = m_dirty_fileinfo.begin(); it != m_dirty_fileinfo.end();) {