TULSA (AP) — A couple's lawsuit contends that an Oklahoma law allowing tribes to intervene in private adoption cases involving Native American children violates their constitutional rights.

The Oklahoma Indian Child Welfare Act permits tribes to interfere even when both birth parents oppose such action and have agreed on who they want as adoptive parents for their child, the Tulsa World reported.

The lawsuit states the couple, who are both 18 and referred to as Jane and John Doe in documents, was upset the Cherokee Nation could intervene with their adoption plan since the birth mother is enrolled in the tribe. It argues they should be afforded the same right to privacy in voluntary adoption cases like people who aren't Native American and claims the state law violates their constitutional rights to due process and equal protection.

The prospective, adoptive parents have joined the couple in the lawsuit, which was filed last week in Tulsa federal court. Oklahoma Attorney General Scott Pruitt and Cherokee Nation Attorney General Todd Hembree are named as defendants.

Chrissi Nimmo, an assistant attorney general for the Cherokee Nation, said the tribe plans to vigorously defend the law. She said the Cherokee Nation is a government and has an interest in what happens to its citizens, just like the state of Oklahoma has an interest in what happens to its residents.

A spokesman for Pruitt said he had not yet seen the lawsuit.

A different case went before the U.S. Supreme Court involving the custody of a young Cherokee Nation girl named Veronica in Oklahoma who was up for adoption.

The girl's birth mother was pregnant when she put the girl up for adoption, and a South Carolina couple had been lined up to receive custody. But the girl's biological father, who was a member of the Cherokee Nation, and his family claimed the federal Indian Child Welfare Act mandated the child be raised within the tribe, and he won custody when the girl was 2.

The U.S. Supreme Court determined in June 2013 that the federal law didn't apply in the case. The Oklahoma Supreme Court lifted an emergency stay that kept Veronica in Oklahoma, and the father handed the girl over to the South Carolina couple.