In VBA I'd like to transfer a ListBox to a function.

For example:
Private Sub CommandButton1_Click()
Call GetIndex(ListBox1)
End Sub

Public Sub GetIndex(TempListBox As ListBox)
MsgBox (TempListBox.ListIndex)
End Sub

Why do I get in VBA (e.g. Excel) at the statement
Call GetIndex(ListBox1)
the Run-time error '13' (Type mismatch)? In real VB it works.

PS: If I use
Public Sub GetIndex(TempListBox As Variant)
it works. But I don't understand why.