Presumptive Republican presidential nominee Donald Trump has promised a physical wall, 35 to 40 feet tall, along the United States’ border with Mexico. Former Texas Gov. Rick Perry (R), who has endorsed the candidate, shot down that idea in an exchange with Snapchat’s Peter Hamby that was posted Monday.

“I’m for Donald Trump, and he says I’m going to build a wall and the Mexicans are gonna pay for it,” Perry said.

“That’s not going to happen,” Hamby said.

“Well, it’s not,” Perry replied. “It’s a wall. But it’s a technological wall. It’s a digital wall. There are some that hear, ‘This is going to be 1,200 miles from Brownsville to El Paso. Thirty feet high.’ And listen, I know you can’t do that.”

But Trump has promised a “real wall.”

“What we’re doing is we have 2,000 miles, right? Two thousand miles. It’s long but not 13,000 miles like they have in China,” Trump said on MSNBC in February. “Of the 2,000, we don’t need 2,000. We need 1,000 because we have natural barriers.”

Trump says Mexico will pay for the wall, an idea government figures in the country have flatly rejected ― including President Enrique Peña Nieto, who reiterated that position on Sunday.

What Perry discussed is more in line with what experts say is a better way to police the border, as well as with what Congress has proposed. A 2013 comprehensive immigration reform bill that passed the Senate, for example, would have mandated 700 miles of fencing. But it also would have ramped up other forms of security and surveillance, such as ground sensors and camera systems.

The government already built about 650 miles of fence along the U.S.-Mexico border, but has not finished the rest for a number of reasons, including the cost. Construction is also difficult because much of the border is in the middle of the Rio Grande river and some of it is on private land.

Perry mentioned some of those complications in an interview in January, referring to Trump’s border wall idea.

“Building a wall ... will take literally years,” he said. “I don’t care how good of a builder you are. And plus the private property rights. This is an issue that really gets complex in a hurry. I know because I’ve had to deal with it.”

Editor’s note: Donald Trump regularly incites political violence and is a serial liar, rampant xenophobe, racist, misogynist and birther who has repeatedly pledged to ban all Muslims ― 1.6 billion members of an entire religion ― from entering the U.S.