Leave a Combobox choice blank

[hulaman]

Hello, I need to be able to have one of multiple comboboxes left blank. I have tried Is Null for one of them with no luck. I need to be able to leave one of the cbo**** choices blank. This select query works perfect except when one is left blank I get nothing back from the query. Here is the code:

'Here is the select statement

strSQL = "SELECT tblSkylights.* " & _
"FROM tblSkylights " & _
"WHERE val(tblSkylights.lngSkySizID)='" & Me.cboSkySize.Value & "' " & _
"AND tblSkylights.strSkyExtruCostID='" & Me.cboMaterial.Value & "' " & _
"AND tblSkylights.strSkyGlasTypID='" & Me.cboGlass.Value & "' " & _
"AND tblSkylights.strSkyFrTypID='" & Me.cboFrame.Value & "' " & _
"AND tblSkylights.strSkyGlasColrID='" & Me.cboOutGlasColr.Value & "' " & _
"AND tblSkylights.strSkyFrColrID='" & Me.cboOutFrColr.Value & "' " & _
"ORDER BY tblSkylights.lngSkySizID;"

Thanks,
chris