Optional argument in a function error

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I'm trying to write a function to retrieve last used cell, but get "Object variable or with block not defined" error...

[VBA]Function GetLastCell(Optional my_sheet As Worksheet)
Dim Cell As Integer

If IsEmpty(my_sheet) Then
GetLastCell = ActiveSheet.UsedRange.Rows.Count
Else
GetLastCell = my_sheet.UsedRange.Rows.Count
End If

End Function[/VBA]

This is what I use to test it:
[VBA]Sub test3()
MsgBox GetLastCell()
End Sub[/VBA]

It works well if I define the argument in the function, but screws me with default value, can anyone help?

[Tinbendr]

My_Sheet can't be optional. You'll have to pass it to be able to use it in the function.

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Thanks, it works.

[mikerickson]

To test if an optional function argument has been passed, one should either use a Variant argument and use IsMissing or should test the argument against a null.
[VBA]Function GetLastCell(Optional my_sheet As Worksheet)
Dim Cell As Integer

If my_sheet Is Nothing Then
GetLastCell = ActiveSheet.UsedRange.Rows.Count
Else
GetLastCell = my_sheet.UsedRange.Rows.Count
End If

End Function[/VBA]