The Carter Center, Human Rights Watch, the International Crisis Group, and other top international charities are supporting a challenge to the constitutionality of a controversial “material support” law designed to cut down on the flow of money and aid to terrorist groups.

The American Civil Liberties Union filed a “friend of the court” brief today in a case before the Supreme Court in behalf of the nine charities, which say the law is overly vague and could restrict even speech and advocacy done with the goal of combating terrorism. The law makes it a crime to provide “material support or resources” to any group that the U.S. government has designated a terrorist organization.

The Supreme Court is scheduled to hear a one-hour oral argument in the case, Holder v. Humanitarian Law Project, this winter. A decision is expected by June, according to the Charity and Security Network.

Under the law, people face up to 15 years in prison for providing “material support” to groups that have been deemed terrorist organizations, even if they oppose the terrorist activities of those groups. “Material support,” as it is defined, could include “personnel,” “service,” “training,” and “expert advice or assistance.”

“The government should not be in the business of criminalizing speech that furthers humanitarian ends,” said Steven R. Shapiro, legal director of the American Civil Liberties Union, in a press release announcing the brief. “No one should face the threat of prison for exercising their First Amendment rights in order to further peace, promote human rights, and provide humanitarian aid around the world.”