Continuous form filtered by combo boxes to show also Null fields

[bakkouz]

Heya,

I have a Continuous form that is filtered by a number of combo boxes, it all works fine and well, But my problem is that when the value in the combo box is Null, the Continuous form resets (shows all records).
Want I want it to do though is show the corresponding fields in the form that are empty.

I'm using the current code:

Private Sub Form_Open(Cancel As Integer)
lngFirstTime = 0
End Sub

Private Sub Combo703_AfterUpdate()
DoFilter
End Sub

Private Sub DoFilter()
Dim fStr As String


fStr = "true "
If Trim(Nz(Combo703, "")) <> "" Then fStr = fStr + "and [Vendor]='" & Combo703 & "' "
Me.Filter = fStr
Me.FilterOn = True
End Sub

help would be much appreciated.
Thanks.

[June7]

Try:
Me.Filter = If IsNull(Combo703) Then Is Null Else "True And [Vendor]='" & Combo703 & "' "