colyseus.js: server error: no available handler for client

I am getting the following error in my NodeJS Program:
colyseus.js: server error: no available handler for "1mnyb2mkjtz2mv9y"

However, I do have handlers attached to the clients. Below is my server-side code:

user.on("connected", attachHandler);

// Attach a handler to the room
function attachHandler(clientID) {
console.log("attaching handler to " + clientID);
gameServer.register(clientID, require('./rooms/mazebattles'));
}

And below is my client-side code where the user connects to the room:

room = client.join(roomID, {create: true}); // Create a new room, and have that client join the room
socket.emit("connected", socket.id);

Colyseus is not supposed to be used along with socket.io. Why would you need socket.io AND colyseus?