Hey @bigtrousers
Try setting a default value for all other fields as well … (starting with the Description field as that sounds like something that has to have a value, like the subject) … and it does not matter whether a field is used on your ticket form or not.
From memory I believe I renamed my Description field to something else (which I cannot remember now of course :-), maybe Message) so just setting a default value for Subject worked for me. That’s probably because I made the other field required on the form.
And that’s the other thing you could try … try making some of the important looking fields on your form as required, so there has to be a value in them.