BERLIN (AP) — Germany’s top court cast doubt Friday on the legality of a key part of the European Central Bank’s rescue plan for the 18-nation eurozone, taking the rare step of asking an EU court for its opinion.

The case involves the so-called Outright Monetary Transactions program, which allows the ECB to buy bonds of ailing eurozone members as an emergency measure. Though the program has never been used, its mere existence has been credited with easing the turmoil in bond markets that was threatening the euro currency bloc.