Did you use 'check part' button in the Library editor? I've never seen this error before so I'm guessing something is not complete in the parttype definition.
I did use 'Check Part' and everything came back as 'OK', but evidently something was wrong. I ended up just starting the part from scratch again, and Viola ! success (so, I must have missed something the first time).
Thanks, for weighing in on the problem.