Vice President Joe Biden told world leaders Tuesday that Donald Trump shouldn’t be taken seriously when he threatens to withdraw US support from NATO.

Biden, meeting in Latvia with leaders of Baltic nations, blasted the Republican presidential candidate for his views on the North Atlantic Treaty Organization.

“The fact that you occasionally hear something from a presidential candidate in the other party, it’s nothing that should be taken seriously because I don’t think he understands what Article 5 is,” Biden said, according to ABC.

Article 5 is “The principle of collective defense is at the very heart of NATO’s founding treaty,” according to NATO’s website. “Collective defense means that an attack against one Ally is considered as an attack against all Allies.”

Last month, Trump suggested he might not be a supporter of Article 5.

“We’re talking about countries that are doing very well. Then yes, I would be absolutely prepared to tell those countries, ‘Congratulations, you will be defending yourself,’” Trump said in an interview with the New York Times.

Trump has called for NATO members to contribute more to the collective defense of treaty signatories.