Hillary Clinton’s campaign on Tuesday released a joint statement from 10 former Obama and Clinton administration diplomats who are questioning Bernie Sanders’ ability to handle foreign policy issues, including working with Iran and fighting ISIS.

The statement comes in the wake of the Vermont senator’s call at Sunday’s debate to normalize relations with the longtime U.S. foe.

“The stakes are high. And we are concerned that Sen. Sanders has not thought through these crucial national security issues that can have profound consequences for our security,” the diplomats said. “His lack of a strategy for defeating ISIS – one of the greatest challenges we face