Combo Box Filter Error

I've searched and tried various methods to this but nothing seems to be working. I'm trying to create a combo box to filter data on a form, but I keep receiving the following error:

Run-time Error '2448':
You can't assign a value to this object.

The combo event procedure I placed is as follows:
Private Sub PtLastNameFilter_AfterUpdate()

Me.Filter = "Last Name = '" & Me.PtLastNameFilter & "'"
Me.FilterOn = True

End Sub

The filtering worked fine for my other combo boxes if the field is numerical:
Private Sub PtMRFilter_AfterUpdate()

Me.Filter = "ID=" & Me.PtIDFilter
Me.FilterOn = True

End Sub

Any help to this would be greatly appreciated. Thank you.