[Error] Invalid playerControllerID

Whenever I try to call SendReadyToBeginMessage() on a client, I get an error in the editor that reads "NetworkLobbyManager OnServerReadyToBeginMessage invalid playerControllerID 255". Other times, the number has been -1, but I have no idea how to make my player's ID valid, I guess. Should I just be setting the readyToBegin flag instead, and checking on the server every once in a while? Or does the readyToBegin flag need to be set as well when calling SendReadyToBeginMessage()?

Any help...

[Error] Invalid playerControllerID