Can I set up a field parameter without checking it is in the feature class?

I've got a script which does some calculations and stores them into fields. I've designed it so that if the fields don't exist they will be created but if they do exist they will be overwritten. When I set up the tool in Arc I tell it that the field parameters are of type field and are derived from the feature class being input.

The problem is, when I run the tool if I enter new field names it stops me with an error because the fields do not exist. I do still want the combo provided from the field type since there are cases where the field will exist though.

I'm thinking it might be a matter of adjusting the validation logic but I've no experience doing that.

Any ideas? Thanks in advance.