Combobox rowsource

I have a userform with three option buttons on it. Depending on which option button is clicked, I would like the rowsource of the combobox to change. I have tried using case select but I get an error that the subscript is out of range. Where am I going wrong?

Private Sub jstselect()
Select Case True

Case OptionButton3.Value
ComboBox2.RowSource = Worksheets("Joist").Range("gg4:gg67")

Case OptionButton4.Value
ComboBox2.RowSource = Worksheets("Joist").Range("gg68:gg147")

Case OptionButton5.Value
ComboBox2.RowSource = Worksheets("Joist").Range("gg148:gg189")

End Select
End Sub