Henderson worked as a chemical engineer for the Wannisky Chemical Corporation. McGuire, Henderson's supervisor, ordered him to remove the labels from several hundred steel drums that had once contained a severely corrosive acid. McGuire told Henderson that they intended to reuse the drums to ship a new chemical fertilizer. Henderson refused to remove the labels, because reusing the old drums would violate both state and federal laws.

When McGuire told another employee to remove the labels and reuse the drums, Henderson reported the company's activities to the state and federal authorities.

Henderson was fired for his refusal to follow orders and for notifying the authorities. In a lawsuit against Wannisky , which legal exception to the employment-at-will doctrine did Henderson use? Explain.