E_CTI_DEVICE_IN_USE Error

I was struggling to figure out why every time I sent a QUERY_AGENT_STATE_REQ to the CTI Server, the CTI logs would show that I got an error which said E_CTI_DEVICE_IN_USE. I knew that the device wasn't in use, so I wondered why in the heck I was getting such a weird error.

It turns out that when I sent my OPEN_REQ to initialize the session, the service mask was missing the Service_Client_Control bit. After I switched it from a 0 to a 4, then it worked out fine. I doubt anyone else will ever have this particular error, but I thought I would note it here in case I ran into it again.