diff --git a/src/os/FileStore.h b/src/os/FileStore.h
index d45dc01a6a8..4ea23b400d7 100644
--- a/src/os/FileStore.h
+++ b/src/os/FileStore.h
@@ -257,6 +257,7 @@ private:
       q.push_back(o);
     }
     Op *peek_queue() {
+      Mutex::Locker l(qlock);
       assert(apply_lock.is_locked());
       return q.front();
     }