How to stop timecards being submitted with blank Payroll Time Type?

Summary

How to stop employees submitting timecards with the Payroll Time Type field being blank, but with hours entered

Content

Hi - hoping someone can give some advice.....

We have our timecards setup so that they bring in the Absences onto the timecard in a read-only format, so employees can see what annual leave etc. they have booked when doing their timecard but they can't actually do anything with it. This is why we haven't got the Payroll Time Type field set to mandatory. 

So how can I setup the timecard to throw up an error message when someone tries to submit a timecard when they've filled in the Hours fields, but not the Payroll Time Type field? At the moment the timecard will submit and go for line manager approval and they can obviously reject it, but it would be useful to cut out that step and not allow timecards to be submitted in the first place!