Reproduced on 2017.3.0f3... perhaps I should update...

I second this. Following code worked fine in 5.5, throws the error above now in 2017.

public class SimpleEmptyMsg : SimpleMsgBase
{
public static SimpleEmptyMsg reusable = new SimpleEmptyMsg();

public static void Command(short msgtype, int channel = Channels.DefaultReliable)
{
//reusable.value = _value;
SendCommand(msgtype, reusable);
}

public static void RPC(short msgtype, NetworkConnection conn = null, int channel = Channels.DefaultReliable)
{
SendRpc(msgtype, reusable, conn);
}

}