diff --git a/src/ca/tcpiiu_IL.h b/src/ca/tcpiiu_IL.h
index 55117a17b..2be897fa2 100644
--- a/src/ca/tcpiiu_IL.h
+++ b/src/ca/tcpiiu_IL.h
@@ -56,7 +56,9 @@ inline SOCKET tcpiiu::getSock () const
 
 inline void tcpiiu::flush ()
 {
+    this->lock ();
     this->flushPending = true;
+    this->unlock ();
     semBinaryGive ( this->sendThreadFlushSignal );
 }