Jake Sullivan, who served as national security adviser in former President Joe Biden’s administration, on Thursday conceded he was surprised to see his former boss’ performance in his 2024 debate against Donald Trump as a new book has resurfaced questions about his mental acuity during his time in the White House.

The June debate hosted by CNN saw Biden struggling to complete sentences and mixing up words, spreading alarm among Democrats and prompting his eventual decision to withdraw from the 2024 presidential contest and cede his place to former Vice President Kamala Harris.

“What happened in that debate was a shock to me, I think it was a shock to everybody,” Sullivan told Politico’s Security Summit.

Sullivan, who defended Biden’s performance as commander-in-chief during the four years he served in the White House, said he could not weigh in on whether it was a mistake for him to pursue reelection, citing his role in the administration.

“I have not weighed in on those issues and would not weigh in on those issues,” he told Politico’s Jack Blanchard.

The former national security adviser also dismissed the reporting included in “Original Sin,” a book co-written by CNN’s Jake Tapper and Axios’ Alex Thompson, which claims Biden referred to Sullivan as “Steve” at least two times in 2022.

“I do not recall that ever happening, and I will tell you Joe Biden knows my name,” Sullivan said. “So I was surprised to read that in the book.”

Sullivan recalled waking up Biden in the middle of the night for major unforeseen developments in international crises, suggesting he was fully competent to serve in those moments.

“I saw him operating, decision-making, executing as commander-in-chief throughout that time,” Sullivan added.

Tapper and Thompson’s book accused Biden and his closest aides of covering up his decline before he inevitably dropped out of the election. The pair detailed an instance in which Biden allegedly did not recognize Oscar-winning actor George Clooney at a fundraiser he hosted for him, and reported his aides’ concerns that he would have to use a wheelchair after he secured reelection.