Wis. Stat. § 806.42: Supplementary general principles of law.

Unless displaced by ss. 806.30 to 806.44, the principles of law and equity, including the law relative to the capacity to contract, principal and agent, estoppel, fraud, misrepresentation, duress, coercion, mistake, bankruptcy or other validating or invalidating causes supplement the provisions under ss. 806.30 to 806.44.