how to resolve error message

Hi All.
I create code that generate copy of a worksheet:

Sub CopySheet()
    Dim ws As Worksheet

    ActiveSheet.Copy After:=Sheets(Sheets.Count)
    Set ws = Sheets(Sheets.Count)

    ws.Name = Format(Date, "mm-dd-yy")
End Sub

It is working. But when I want to create a new sheet on the same date, in this case, the sheet will be created but an error message will pop up. How to resolve this error message?

Thanks