A proposed outline of a deal that President Donald Trump and North Korean leader Kim Jong Un may sign during their Vietnam summit this week would be “a bad deal for the United States.”

That’s not the view of a cynical expert, or a Democrat. It’s the view of a top national security adviser to South Korean President Moon Jae-in.

Trump and Kim are in Hanoi to discuss how to strike a deal to dismantle North Korea’s nuclear program. Negotiators are currently working with an outline of a deal first described to Vox by three knowledgeable sources on Tuesday, where the US would lift some sanctions on North Korea in exchange for a commitment from Kim to stop nuclear-fuel production at a key nuclear facility.

Both leaders would also sign a peace declaration to end hostilities toward each other and open liaison offices in both countries, and North Korea would return the remains of a yet-unspecified number of missing US troops who fought and died in the Korean War.

But when asked publicly about the Vox-reported outline of the pact by the event’s moderator, Moon Chung-in, a special adviser for foreign affairs and national security to South Korea’s president, was not impressed.

“It’s a good deal for North Korea and a bad deal for the United States,” he said during a Tuesday speaking engagement at the Korea Economic Institute in Washington. He went on to say what he’d prefer to see out of Hanoi is a roadmap for negotiations between Washington and Pyongyang going forward and Yongbyon — the key North Korean nuclear facility — completely shutdown.

When I followed up with him after the session, Moon reiterated his point about the tentative outline. “It’s not a very good deal,” he told me. For all the incentives the US might give North Korea, “the US can get much more than that,” he said.

I emailed Moon Tuesday night and asked him to clarify once more. Here’s part of what he wrote back on Wednesday morning.

“Our government cannot accept the idea of exchanging a simple freeze of nuclear activities for such hefty incentives as peace declaration, liaison office, and relaxation of sanction,” he said. “I was only criticizing the hypothetical outline, especially idea of freeze,” he added, noting that if the moderator had said the deal included closing Yongbyon down, which as of now is not in the tentative deal outline, “my answer should have been different.”

Why Moon Chung-in’s statement matters

There’s still a long way to go before Trump and Kim put pen to paper, and of course the deal could change dramatically between now and then. But Moon’s statements are still shocking — and here’s why.

Moon Chung-in’s boss, Moon Jae-in (no relation), has been a fervent champion of the US and North Korea’s attempts to end the years-long nuclear standoff, and he’s consistently asked both parties to concede more to one another.

For one of his top advisers to trash the tentative deal is more than surprising, because it shows that the South Koreans could break ranks with the US if the final deal isn’t received well in Seoul. It’s unclear as of now if the South Korean president agrees or disagrees with his aide, though.

Moon Chung-in did hedge toward the end of his public remarks. In response to another question about what might constitute a bad deal for South Korea, he said “there is no such thing as a bad deal,” indicating that as long as US forces don’t leave South Korea and negotiators don’t leave the room screaming, any diplomatic accord would prove a sign of progress between the US and North Korea.

Still, a top adviser of a critical ally in the US-North Korea talks just trashed the general outline of what Trump and Kim may sign in two days. That’s worth keeping in the back of your mind as they work toward a final agreement.

UPDATE: This piece has been updated to include Moon Chung-in’s emailed response to me. It has also been corrected to reflect that the moderator asked Moon about a freeze in nuclear-fuel production at Yongbyon, not the dismantlement of the facility.