Cascading Combo box doesn't update 2nd box options

I am using MS Access 2010 to store records about audits that take place at our hospital. I am trying to create a form where it will filter the audits according to the drop down options selected.

I want the Specialty combo boxes to cascade from Directorate, so if I select "Family and Public Health" Directorate, only the Specialties that are within that Directorate appear in the combo box below.

My ComboDirectorate Row Source is SELECT [tblDirectorate].[Directorate Key], [tblDirectorate].[Directorate] FROM tblDirectorate;

My ComboSpecialty Row Source is SELECT tblSpecialty.[Specialty Key], tblSpecialty.Specialty FROM tblSpecialty;

I then have After Update code for ComboDirectorate:

Private Sub ComboDirectorate_AfterUpdate()
Me.ComboSpecialty.RowSource = _
  & "SELECT [tblSpecialty].[Specialty Key], [tblSpecialty].[Specialty]" _
  & "FROM tblSpecialty WHERE [tblSpecialty].[Directorate] = " _
  & Me.ComboDirectorate
Me.ComboSpecialty.Requery
End Sub

This doesn't seem to have any affect on the specialties listed.

I have tried putting in ' ' as per Can't get the cascading combo boxes to work

WHERE Directorate = '" & Nz(Me.ComboDirectorate) & "' " & _

But it still doesn't list the specialties according to the directorate selected.

Solution

Using June7's answer I coded the following:

Private Sub ComboSpecialty_GotFocus()
  If IsNull(Me.ComboDirectorate) Then
     Me.ComboSpecialty.RowSource = "SELECT [Specialty Key], Specialty FROM tblSpecialty"
 ElseIf Not IsNull(Me.ComboDirectorate) Then
     Me.ComboSpecialty.RowSource = "SELECT [Specialty Key], Specialty FROM tblSpecialty WHERE Directorate = [ComboDirectorate]"
 End If

Me.ComboSpecialty.Requery
End Sub

If the user doesn't select a value from the first combo box (ComboDirectorate) then all values are displayed in the second combo box (ComboSpecialty). If a value is selected in the first box then only related values are displayed in the second combo box.