RTG "Is missing a source."

The string, "Is missing a source" is the entire content of a warning from craft validation. I am alerted to this warning whenever I try to launch the craft. Clicking on the message highlights the RTG fitted to this craft.

At best, this is a bad message: what "source" does the RTG need? I can't imagine what it might be because I'm not aware of real-life RTGs requiring anything at all. The source of their extreme reliability is that they require no moving parts. This means there cannot be a pump, therefore nothing could be pumped into an RTG, so what "source" could an RTG possibly require?

Guessing a little, if the message means the RTG needs a connection to a battery, then the root cause may be the same as the bug I filed about a fuel cell not charging a battery; i.d. ME30hi