Student-Dependent IHS Expiry Date?

My fiance and I currently are applying for her Student-Dependent visa.
On the page where it asks my fiance for her details for the immigration healthcare surcharge, it requires her to enter my visa expiry date. As I do not yet have a visa and we are applying together, we are not sure what to enter in this field, but it is required and we cannot leave it blank.
Should we enter the end date of my course? I am not sure how to proceed.