Wis. Stat. § 891.25

When any written instrument constitutes the subject of the action or proceeding or when the signing of such instrument is put in issue and the instrument purports to have been signed, the instrument itself is proof that it was signed until denied by the oath or affidavit of the person by whom it purports to have been signed or by a pleading. This section does not extend to an instrument purporting to have been signed by a person who died before proof is required.