I think I've spotted why. It's moving the record to the first, but can't because there are no records. Try this:
Code:
Dim intRecordCount As Long
intRecordCount=rs.RecordCount

If intRecordCount => 1 Then
 rs.MoveFirst 
 Do Until rs.EOF 
  cmbUsers.AddItem rs.Fields("UserID") 
  rs.MoveNext 
 Loop
End If
Hopefully that works!