diff --git a/constant.go b/constant.go
index 492dd70..ae68bf5 100644
--- a/constant.go
+++ b/constant.go
@@ -3,6 +3,8 @@ package f5
 const (
 	IM_HTTP_CLI_MGR_RESPONSE = iota + 1
 	IM_EXEC_MAIN_THREAD_CB
+
+	IM_CUSTOM_MSG_BEGIN = 100
 )
 
 const (