Reference fieldlist of parent form from subform

I have a form that populates some data from a query. On that form is a subform. This subform has a listbox that needs one of the results from the parent form's query.

For example, my parent form does: "SELECT PK_Person, Name, Age FROM Persons", and uses the Name and Age fields to populate data on the form.
The subform needs access to the PK_Person value in the parent form.

I tried Me.Parent.PK_Person, but that didn't work.

How do I access this from the subform?

thanks