Prosecutors say there's nothing unusual about a court order requiring Baltimore Police Officer William Porter to testify against fellow officers in the Freddie Gray case.

Assistant Attorney General Carrie Williams made the argument on behalf of city prosecutors in a filing Wednesday in the Court of Special Appeals, Maryland's second-highest court.

Porter is appealing a Baltimore Circuit Court ruling that he must testify against Officer Caesar Goodson and Sgt. Alicia White.

Porter contends he could still be exposed to perjury charges, since prosecutors have said he lied on the witness stand at his own manslaughter trial stemming from Gray's death. That trial ended in a hung jury in December.

A hearing on the appeal is set for March 4.