I'm not sure if that's the case because when i change .tag to .caption (as below) then I start getting the runtime error 13 again.

Like I'd said earlier, the same code snippet work worked as a 'standalone base convertor' however when I integrated it into my scientific calculator code it just refused to run despite the fact it's EXACTLY the same code.

Private Sub optNumber_Click(Index As Integer)
Dim OldNumber As String
Dim NewNumber As String

OldNumber = txtNumber.Text

'NewBase = optNumber(Index).Tag

If optNumber(Index).Caption = "" Then

'do nothing

Else

NewBase = optNumber(Index).Caption

End If

(note changing back from .Caption to .Tag causes the runtime error to disappear, however when the radio buttons (optNumber are clicked, nothing happens)