Prisoners rioted at the Iowa State Penitentiary and held prison staff members as hostages. The warden agreed in writing that no reprisals would be levied against the rioting inmates. In exchange, the prisoners released the hostages. After the hostages were released, several of the prisoners were punished for the riot. One prisoner, Wagner, was placed in solitary confinement for 30 days. He also received 180 days of administrative segregation and the loss of 1,283 days of goodtime earned. On what legal grounds could the warden refuse to keep his promise to the inmates? Explain. Wagner v. State, 364 N.W.2d 246 (IA).