invalid use of null....using combo box in vb6

[vb6 student]

My combobox in vb6 is to reterievr values from access database on form load event. But it gives this error of " invalid use of null "despite of so many values present in database of that particular field asked
I am copying my code........... .some expert kindly help me



Dim cn As New ADODB.Connectio n
Dim rs As New ADODB.Recordset
Private Sub Form_Activate()

cn.ConnectionSt ring = "provider=micro soft.jet.oledb. 4.0;" & " data source=" & App.Path & "\Database. mdb"
cn.Open
rs.Open "select distinct NOUN_DESCRIPTIO N from ITEM", cn, adOpenDynamic, adLockReadOnly
Do While rs.EOF <> True
Combo2.AddItem rs!NOUN_DESCRIP TION
Exit Do
rs.MoveNext
Loop
End Sub