The American Civil Liberties Union is challenging the constitutionality of charges brought against Republican Sen. Larry Craig for soliciting sex in the men’s bathroom of a Minneapolis airport.

The ACLU filed a 12-page brief with the Minnesota Court of Appeals that states:

“The law the state has applied to this defendant makes it a crime to use offensive language, and since the use of offensive language alone cannot be made a crime, the law is unconstitutionally overbroad on its face.”

The ACLU also takes issue with the “secret sting” police conducted that ensnared Craig. Instead, the legal group recommends authorities use posted signs to deter people from having sex in public.

The brief claims: “More often than not, a secret sting is far less carefully tailored to furthering an interest in deterring public sex than a posted sign warning that the premises are patrolled. A posted sign is far less likely to risk ensnaring invitations to have private sex than a secret sting. At the same time, a posted sign is not only as likely to deter public sex as, but indeed far more likely to deter public sex than, a secret sting, whose existence is known only to those who are arrested and not to those who follow in their wake.”

Craig was arrested on June 11 because, police have charged, Craig extended an invitation for sex through various signals to an undercover police officer. Craig pled guilty to the charges, but later claimed he was misunderstood.