Wis. Stat. § 631.48: Nonwaiver clause.

An insurer may insert in any insurance policy a provision that no change in the policy is valid unless approved by an executive officer of the insurer, or unless the approval is endorsed on the policy or attached to it, or both, and that no agent has authority to change the policy or waive any of its provisions. This does not preclude a person claiming a right under the policy from relying on waiver or estoppel in an appropriate case.