Thanks for pointing this out! That limitation is largely a holdover from when the Kafka sink was written and our support for accepting multiple data types was not as good as it is now. As things stand today, it should be a pretty simple change to enable this.

I'll go ahead and open an issue to get that addressed, but in the future please feel free to do so yourself for anything that's tripping you up! We really value this kind of feedback and try to address it as promptly as possible.