The news that Donald Trump Jr. invoked attorney-client privilege Wednesday to refuse to testify about a key conversation with his father was met with confusion and some amount of mockery.

As House Intelligence Committee Vice Chair Adam Schiff (D-CA) put it, that privilege doesn’t typically protect “a discussion between father and son,” and shouldn’t stop Trump Jr. from discussing a phone call they had about negotiating the fallout from news reports over his June 2016 meeting with a Kremlin-linked attorney.

Legal experts told TPM that there are specific situations in which Trump Jr.’s invoking the privilege would be legitimate, though they noted that publicly reported information on the circumstances of the call suggests that it was not. In any event, the Republican-controlled House committee is unlikely to compel the GOP President’s son’s testimony via subpoena. Special Counsel Robert Mueller, on the other hand, will surely want to know the details of this call, and will be less sympathetic in his pursuit of that information, the experts said.

“If you’re Bob Mueller and you get an interview of Donald Trump Jr., this is going to be one of the three or four main topics in your outline. Like at the Roman numeral level,” said Andy Wright, a former White House associate counsel under President Barack Obama.

“This strategy would not work in a grand jury or special counsel investigation unless it was a valid invocation of attorney-client privilege; whereas in Congress, it’s based more on congressional will,” Wright, who is now a professor at Savannah Law School, added, noting that Trump Jr.’s refusal to talk to Congress “may be effective in a practical way even if its legally defective.”

Trump Jr.’s attorney, Alan Futerfas, did not respond to TPM’s request for comment. Trump’s White House special counsel Ty Cobb declined to comment on the record.

At issue for the Russia probe investigators is what Trump Jr. said to his father after the New York Times broke the news that Trump Jr. met with Natalia Veselnitskaya, a Russian lawyer who Trump Jr. was told had “dirt” on Hillary Clinton, during a June 2016 sit-down at Trump Tower. The Washington Post reported that the President, while traveling back from the G-20 summit on July 8, 2017 on a plane full of aides, “dictated” a misleading statement released by Trump Jr about the purpose of the meeting. That statement said the meeting focused mostly on the ability of Americans to adopt Russian children.

Lawyers for both Trump and his son were on the call hammering out that response, according to the Wall Street Journal.

But the presence of attorneys alone does not validate Trump Jr.’s claim of attorney-client privilege.

Legal experts told TPM that the eldest Trump son’s legal team would need to be engaged in a common interest agreement with his father’s lawyers to openly share information; that the conversation would have to be specifically focused on obtaining legal advice rather than political crisis communications; and that no non-privileged parties could be listening in on the call.

“If Trump is talking on a plane full of people, that’s not a privileged conversation even if his attorney was there,” said Asha Rangappa, a former FBI special agent who now serves as a senior lecturer at Yale University’s Jackson Institute for Global Affairs. “He’s not representing everyone on the plane.”

It’s not entirely clear who else was involved with this specific call. Trump Jr. has acknowledged coordinating his statement with adviser-turned-communications director Hope Hicks, who was aboard Air Force One and speaking with Trump while the statement was being drafted, according to CNN.

Former federal prosecutor Steve Miller noted that the “crime-fraud exemption” could also come into play if either attorney was providing guidance on how to conceal the true purpose of the Trump Tower meeting.

“If legal advice is being sought or legal communications exist for the purpose of furthering a fraud, then it’s not privileged,” he said.

Getting to the bottom of these questions is more likely to be a job for Mueller’s team than for Congress, given that it is at the discretion of a committee chair to decide whether to accept a privilege claim. Chairman Mike Conaway (R-TX) so far appears uninterested in following up on his Democratic colleagues’ calls to subpoena Trump Jr.

“A lot of questions were asked and answered, and from my perspective all of our questions were answered,” Conaway said after the seven-hour interview wrapped.

As Rangappa put it, “I think that what was really going on with Don Jr. is that he was buying time. When he goes in to testify he’s testifying under oath so he’s getting locked into what he says. And anything he says that could later get disproven would make him liable for perjury.”

Trump Jr.’s attorney has reportedly requested more time to study whether they will ultimately voluntarily offer more facts about the father-son call. No deadline has yet been set to produce that information.