( MAADDI, AP ) The NFL players’ union has appealed the league’s suspension of Adrian Peterson.

In a letter to its player representatives and executive committee obtained by The Associated Press on Thursday, the NFLPA calls Commissioner Roger Goodell’s punishment “unprecedented, arbitrary, and unlawful” and demands an independent, neutral arbitrator hears Peterson’s case.

Goodell on Tuesday suspended Peterson without pay for the rest of the season and told him he will not be considered for reinstatement before April 15 for his violation of the NFL personal conduct policy. Peterson pleaded no contest Nov. 4 to misdemeanor reckless assault in Texas for injuries to his 4-year-old son with a wooden switch.

The union says Peterson was told the games he missed on a special exempt list would count as time served toward a suspension. The 2012 NFL MVP played just one game for the Minnesota Vikings this season.