filter query with custom field

I created a query from a table and added a field that checks another field for a specific character at a specific position. I entered the following expression into the field box in design view:

ZorA: IIf(Mid$([MoldCode],4,1)="A","Al","Zn")

This works fine, but my problem is that when I try to filter this column, access does not find any matching criteria and locks up.

Does anyone know why this won't work?