I'm stumped!!

I am trying to populate a Combobox from a range of data. Using the code
below, I can fill a listbox. However, If I add ComboBox1 to the form and
change the code below from "Me.ListBox1.AddItem cell.Value" to
"Me.ComboBox1.AddItem cell.Value", I get an error: Runtime error 70
Permission denied. Can someone help?

Private Sub UserForm_Initialize()
Dim cell As Range
Dim rng As Range

With ThisWorkbook.Sheets("shOrder")
Set rng = .Range("a4", .Range("a4").End(xlDown))
End With

For Each cell In rng.Cells
Me.ListBox1.AddItem cell.Value
Next cell
End sub

--
Steve