President Donald Trump’s assertion that Germany owes vast sums of money to NATO and the United States suggests a lack of understanding of how the military alliance works, according to a former US ambassador to NATO.

On the heels of a visibly awkward meeting with German Chancellor Angela Merkel, Trump tweeted Saturday that Germany owed vast sums to NATO and demanded the US be paid for its “very expensive” defense of the European ally.

According to NATO’s official guidelines, member states are required to spend at least 2 percent of their GDP on the alliance. However, most members fail to meet that target, and ultimately contributions are based on each country’s capability.

That means technically no country owes anything to NATO or other member states as Trump suggested.

“Sorry, Mr. President, that’s not how NATO works,” said Ivo Daalder, who was the US representative to NATO from 2009 to 2013.

“The US decides for itself how much it contributes to defending NATO. This is not a financial transaction, where NATO countries pay the US to defend them. It is part of our treaty commitment,” he wrote on Twitter.

Daalder agreed with the notion that European allies should contribute more to NATO “but not as favor (or payment) to the US.”

Likewise, he added that America’s “large military commitment” to NATO was “not a favor to Europe” but was “vital for our own security.”

Trump has repeatedly called on US allies to contribute their fair share to NATO, which he has also criticized as obsolete.

In a second tweet on Saturday, Trump attacked the “fake news” coverage of his public appearances with the German leader, which some reports described as awkward and frosty.

“Despite what you have heard from the FAKE NEWS, I had a GREAT meeting with German Chancellor Angela Merkel,” the president wrote.

During their first meeting in the Oval Office on Friday, Trump did not respond to multiple requests from photographers that the two leaders stage a handshake for cameras.

Merkel looked visibly baffled by some of Trump’s comments during a joint press conference in the White House.

Many nations owe vast sums of money from past years and it is very unfair to the United States,” Trump said. “These nations must pay what they owe.”

“During our meeting, I thanked Chancellor Merkel for the German government’s commitment to increase defense spending and work toward contributing at least 2 percent of GDP,” the president added.

Trump has previously suggested that US defense of a NATO ally would depend on the member’s contributions to the alliance.