diff --git a/lualib/skynet.lua b/lualib/skynet.lua
index 4f1ae174..cfa23fb9 100644
--- a/lualib/skynet.lua
+++ b/lualib/skynet.lua
@@ -633,6 +633,9 @@ function skynet.ret(msg, sz)
 	local tag = session_coroutine_tracetag[running_thread]
 	if tag then c.trace(tag, "response") end
 	local co_session = session_coroutine_id[running_thread]
+	if co_session == nil then
+		error "No session"
+	end
 	session_coroutine_id[running_thread] = nil
 	if co_session == 0 then
 		if sz ~= nil then
@@ -641,9 +644,6 @@ function skynet.ret(msg, sz)
 		return false	-- send don't need ret
 	end
 	local co_address = session_coroutine_address[running_thread]
-	if not co_session then
-		error "No session"
-	end
 	local ret = c.send(co_address, skynet.PTYPE_RESPONSE, co_session, msg, sz)
 	if ret then
 		return true