diff --git a/lualib/skynet.lua b/lualib/skynet.lua
index 7b4c466a..202e4964 100644
--- a/lualib/skynet.lua
+++ b/lualib/skynet.lua
@@ -34,7 +34,7 @@ skynet.cache = require "skynet.codecache"
 function skynet.register_protocol(class)
 	local name = class.name
 	local id = class.id
-	assert(proto[name] == nil)
+	assert(proto[name] == nil and proto[id] == nil)
 	assert(type(name) == "string" and type(id) == "number" and id >=0 and id <=255)
 	proto[name] = class
 	proto[id] = class