Pass Listbox by reference

Hello,

   I have a list box called "lst_AddIPP_AvailableParts" which I want to pass by reference to a subroutine when the listbox is clicked. The subroutine enables or disables a button if there are any items in the listbox that are selected. The listbox has the multiselect property set to True. The calling code is below:

Private Sub lst_AddIPP_AvailableParts_Click()    
    EvaluateListClick lst_AddIPP_AvailableParts, btn_AddIPP_PartNLeft
End Sub

The function that evaluates the listbox, "EvaluateListClick" is defined below:

Private Sub EvaluateListClick(ByRef mylist As Access.listbox, ByRef mybtn As Button)
    Dim bck As Boolean
    Dim varItm As Variant
    bck = False
   
    For Each varItm In mylist.ItemsSelected
        bck = True
        Exit For
    Next varItm
   
    If bck = True Then
        mybtn.Enabled = True
    Else
        mybtn.Enabled = False
    End If
End Sub

The form crashes when the user clicks the listbox and the error message says: "Type mismatch". The debugger says the listbox passed to "EvaluateListClick" has a value of null. Interestingly if I use the listbox on the line above the "EvaluateListClick" call, it works fine. Also fine works when I pass the listbox as a parameter to a subroutine when a button is clicked. The only place I get this error is when I try to pass the listbox as a parameter from within the listbox event.

Not sure how to get rid of this problem. Can someone help?

To replicate, place a listbox on a form and add an onClick event. Use the above code for the implementation.
The listbox has the multiselect property set to True.

Cheers,
Max.