Assist - Oops something went wrong - Missing slot list {room}

Just in case anybody else has the same problem … I was trying out “turn on the kitchen light” with the new Assist integration and was getting “Oops something went wrong”, and the log included “Missing slot list {room}”.

First, I realised I had not put the lights in the kitchen area, as I haven’t had a need for areas before. Once I did that I was still getting the error and log, so I restarted HA for good measure. At some point it just started working.

I added other lights to a different area that previously had no entities and Assist worked straight away.