Former President Trump challenged President Biden to a cognitive test in a speech Saturday, but forgot the name of the doctor who gave him his own similar test.

Trump said in Detroit that “Doc Ronny Johnson” gave him a cognitive test that he “aced.” The doctor’s name is Rep. Ronny Jackson (R), who has since been elected to Congress in Texas.

The gaffe underlines age concerns about both candidates among voters. Either Biden or Trump would be the oldest president in U.S. history at the end of a second term, and both have faced attacks about their memories and physical and mental fitness.

The former president underwent a standard cognitive test in 2018, an assessment asking a patient to read and recall a set of words, identify shapes and other common tasks. The test is designed to detect early signs of Alzheimer’s disease. Trump went on a media tour bragging about his performance in 2020.

Trump, 78, has repeatedly held up his mental prowess, despite a shaky track record of mistakes mixing up names. Some of the country’s largest business leaders were “shocked” at Trump’s apparently rambling speech at a private meeting last week, CNBC reported.

Biden, 81, has his own history with name mix-ups. Nearly 90 percent of Americans believe the president is too old for a second term, according to a survey in February. More than 50 percent of survey respondents said both he and Trump are too old.

The president has mostly brushed off concerns about his age, telling voters that his ideas are still young.

Trump and Biden are neck and neck in polls of the presidential election. Trump leads Biden nationally by less than 1 percentage point, according to The Hill/Decision Desk HQ average of polls.

The Hill has reached out to the Trump campaign for comment.