Hidden required select field with default value

Hello,

 

I'm trying to modify a form where I'm hiding a select field.

 

This field is usually required.

 

When setting the #type = hidden

The field is hidden, but when trying to submit the form I get an error about
the fields being required.

Looking at the $form dump, I see that the select field has a default value.
(a term id). 

I tried changing the Required to false when hiding but then the field looses
its value.

 

Is there some other property I need to set?

 

Thanks

 

Idan