Fill Listbox: AddItem

I am trying to get a range from a worksheet and add it to my listbox1 on a form but i keep getting this error message.. Run=time error '1004': Method 'Range' of object '_worksheet' failed

Dim ws As Worksheet
Set ws = Worksheets("iSheet")
Dim ilastrow As Integer
ilastrow = ws.Range("A65536").End(xlUp).Row
Dim irow As Integer
irow = 0

For irow = 0 To ilastrow
If Trim(ws.Range("a" & irow).Value) <> "" Then
With ListBox1
.AddItem Trim(ws.Range("a" & irow).Value) & " - " & Trim(ws.Range("b" & irow))
End With
End If
Next