Does FindFriends only work when not in a room?

I have FindFriends working when I am not in a room, and I call it periodically to get the online status of friends. Works fine before I join a room.

However when I create/join a private (IsVisible = false) room, it doesn't work anymore. I would like to invite friends to the private room once I'm already in it. I get these two errors instead:

Operation failed: OperationResponse 222: ReturnCode: -2 (Unknown operation code). Parameters: {} Server: GameServer
UnityEngine.Debug:LogError(Object)
NetworkingPeer:OnOperationResponse(OperationResponse) (at Assets/Photon Unity Networking/Plugins/PhotonNetwork/NetworkingPeer.cs:1544)
ExitGames.Client.Photon.PeerBase:DeserializeMessageAndCallback(Byte[])
ExitGames.Client.Photon.EnetPeer:DispatchIncomingCommands()
ExitGames.Client.Photon.PhotonPeer:DispatchIncomingCommands()
PhotonHandler:Update() (at Assets/Photon Unity Networking/Plugins/PhotonNetwork/PhotonHandler.cs:157)

FindFriends failed to apply the result, as a required value wasn't provided or the friend list length differed from result.
UnityEngine.Debug:LogError(Object)
NetworkingPeer:OnOperationResponse(OperationResponse) (at Assets/Photon Unity Networking/Plugins/PhotonNetwork/NetworkingPeer.cs:1922)

In NetworkingPeer line 1906, both lists are empty:
i.e. onlineList and roomList are both null.