error message macro userform combobox

Hi,

I want the user to select a textstring in a combobox that should be displayed in range("I2"). Somehow i get an error message. What am i doing wrong?

Sub Tekstvak2_Klikken()
Opvoerder.Show
End Sub

Private Sub OpvoerderOptionButton1_Click()

sheet1.Activate
Cells(2, 9).Value = OpvoerderCombobox.Value

End Sub

Private Sub OpvoerderOptionButton2_Click()
Unload Me
End Sub

Private Sub UserForm_Initialize()
OpvoerderCombobox.Clear
With OpvoerderCombobox
    .AddItem "WK"
    .AddItem "MB -D"
    .AddItem "AM"
    .AddItem "AB"
    .AddItem "BK"
    .AddItem "EL"
    .AddItem "AJM"
End With
End Sub