Cannot close request if no sub-category defined

I installed the trial version of Service Desk Plus v7 yesterday and it worked first time, easiest installation of anything involving MySQL that I've done. Fantastic.

Anyway, I was testing the the email fetching functionality and sent an email to the helpdesk which was received and allocated a category of General. The General category didn't have any sub-categories or items defined.

Today, whilst customising the helpdesk I made the sub-category field on the request form mandatory without thinking it through and then went to close the test emailed request where I was told that sub-category was mandatory.

I then removed the mandatory setting for the sub-category, tried again and was given the same error. Logged out, logged in, no difference. Eventually I had to go in and add a sub-category to the General category in order to close the request.

Is this a known problem or have I just got things muddled up?

Thanks

Jonathan