“Message field is required.”

I’m logged in as an authenticated user, and I browse to a user’s profile page, and I click the “Contact” tab to send them a message ...

I type “Test” in the subject line.
I type “Test” in the message area.
(I also answer the Captcha challenge.)
I click submit.

I get this:

“Message field is required.”

When ... there IS content (albeit very LITTLE) in the message field. Is there a setting somewhere which requires a minimum amount of user input in a field and forces that minimum input before accepting it?