Recordset Update From Combo

I want to update a field (field1) in each record of a recordset (qryUpdate)

I got the following code (in green) working fine - so far so good. I actually need to update the field from a selection from an unbound combo box. This looks up a table with two fields Period ID (autonumber PK) and a text field called Quarter. I've tried various combinations including the code in red and code such as
"rst![field1] = Me.cboPeriod"
"rst![field1] = Me.cboPeriod.Column(0).value" etc

but can't get it to work. It has to be something simple but I can't see it.


Private Sub cboPeriod_AfterUpdate()
Dim rst As DAO.Recordset

Set rst = CurrentDb.OpenRecordset("qryUpdate")
Do Until rst.EOF

If IsNull(rst![field1]) Then
rst.Edit
rst![field1] = 5
rst.Update
End If

rst.MoveNext
Loop
End Sub




Private Sub cboPeriod_AfterUpdate()
Dim rst As DAO.Recordset
Set rst = CurrentDb.OpenRecordset("qryUpdate")

Do Until rst.EOF

If IsNull(rst![field1]) Then
rst.Edit
[rst![field1] = Me.cboPeriod.Column(0)
rst.Update
End If

rst.MoveNext

Loop

End Sub