Is it time to B!ng?

Custom Document Library Item check-in throws error

I got the miserable "Value does not fall within the expected range" error after checking in an item in my newly created custom document library with a custom content type.

Since the cause of the error "Didn't fall within my knowledge range" I had to google ... and *bing!* found what I was looking for.

Lesson learned: Do not set the "required" attribute to true in your content type definition.