diff --git a/tsd_exclusive.h b/tsd_exclusive.h
index 9d03773..2383674 100644
--- a/tsd_exclusive.h
+++ b/tsd_exclusive.h
@@ -63,7 +63,9 @@
     // We don't have a way to iterate all thread local `ThreadTSD`s. Simply
     // drain the `ThreadTSD` of current thread and `FallbackTSD`.
     Instance->drainCache(&ThreadTSD);
+    FallbackTSD.lock();
     Instance->drainCache(&FallbackTSD);
+    FallbackTSD.unlock();
   }
 
   ALWAYS_INLINE void initThreadMaybe(Allocator *Instance, bool MinimalInit) {