diff --git a/lualib/skynet.lua b/lualib/skynet.lua
index 1585c391..9cb7c748 100644
--- a/lualib/skynet.lua
+++ b/lualib/skynet.lua
@@ -98,6 +98,9 @@ do ---- avoid session rewind conflict
 
 	-- in dangerzone, we should check if the next session already exist.
 	local function checkconflict(session)
+		if session == nil then
+			return
+		end
 		local next_session = session + 1
 		if next_session > dangerzone_up then
 			-- leave dangerzone