add

@@ -68,8 +68,13 @@ end
 local args=ngx.req.get_uri_args()
 local method=args["method"]
 if method == "list" then
-  local u_name = args["upstream"]
-  list(u_name)
+  if not upstream then
+    ngx.exit(400)
+    ngx.log(ngx.ERR, "request params is error. upstream is null")
+  else
+    local u_name = args["upstream"]
+    list(u_name)
+  end
 elseif(method=="down" or method=="up") then
   local upstream=args["upstream"]
   local backup=args["backup"]=="true" and true or false