Donald Trump apparently thinks that talking to world leaders is like picking up women. No, he’s not grabbing them by the, well, hopefully anywhere. But he has been handing out his private cell phone number to world leaders—including Canadian Prime Minister Justin Trudeau, who went ahead and used it. There are some reasons for concern:

Presidents generally place calls on one of several secure phone lines, including those in the White House Situation Room, the Oval Office or the presidential limousine. Even if Trump uses his government-issued cellphone, his calls are vulnerable to eavesdropping, particularly from foreign governments, national security experts say.

“If you are speaking on an open line, then it’s an open line, meaning those who have the ability to monitor those conversations are doing so,” said Derek Chollet, a former Pentagon adviser and National Security Council official now at the German Marshall Fund of the United States.

In fact:

“If you are Macron or the leader of any country and you get the cellphone number of the president of the United States, it’s reasonable to assume that they’d hand it right over to their intel service,” said Ashley Deeks, a law professor at the University of Virginia who formerly served as the assistant legal adviser for political-military affairs in the U.S. State Department.

Also, we know what happens when Trump talks to world leaders with a full complement of official babysitters, from temper tantrums to blurting out classified information. The prospect of him just wandering around talking to the heads of other states in the wild, with no witnesses or people to try to rein him in is downright terrifying. And of course a Justin Trudeau or any other top official who calls Trump on his private, unsecured number absolutely will have their staff on hand to record what’s said and help them come out on top.

But her emails!