A connection has already been set as ready. There can only be one.

I got the error below.
A connection has already been set as ready. There can only be one.
UnityEngine.Networking.NetworkIdentity:UNetStaticUpdate()

However, I tried to clear ready flag by invoke script below, still got the error don't know why.
NetworkServer.ClearLocalObjects ();
NetworkServer.ClearSpawners ();
NetworkServer.SetAllClientsNotReady ();
this.matchMaker.DestroyMatch (matchNetworkID, OnConnectionDrop);

Any idea?