The Washington Post's fact-checker flunked Bernie Sanders Thursday for saying his rival Hillary Clinton believes he is "not qualified" to be president, even though the Vermont senator's source of that claim was a Post story that said the Democratic front-runner questioned whether he "is qualified" to serve.

The newspaper's Juliet Eilperin and Anne Gearan authored an article this week titled, "Clinton questions whether Sanders is qualified to be president."

"Former Secretary of State Hillary Clinton on Wednesday questioned whether her rival in the Democratic presidential primary … is qualified to be president," they reported.

That story was based on an MSNBC interview Clinton gave, in which she never used any variation of the word "qualified" at all. She did say, however, that Sanders "hadn't done his homework" on financial regulations, and those comments were summarized in the Post story.

Sanders' campaign said he drew from that story, and others, when he went on to say this week that Clinton is unfit to be president.

"She has been saying lately that she thinks that I am, quote unquote, not qualified to be president," the senator said in Philadelphia. He added later, "They're going to question my qualifications, well I'm going to question theirs."

But now the Post's fact checker is saying the Vermont senator is off base, apparently because Sanders didn't read the Post story closely enough.

Clinton never said the senator was "not qualified," the paper's fact-checker, Glenn Kessler, wrote.

Further, neither the Post story nor its headline made this claim. Instead, the reporters themselves used the word "qualified" when they wrote that Clinton had questioned Sanders' credentials.

As a result, Sanders failed the fact-check.

"Sanders is putting words in Clinton's mouth. She never said 'quote unquote' that he was not qualified to be president," Kessler explained.

But at the same time, Kessler excused the Post for summarizing Clinton's remarks by saying she was questioning whether he "is qualified."

"Headline writing is an imperfect art. The editor often has to summarize the meaning of a complex and nuanced article in just a few words. Many Washington-based reporters have experienced the frustration of having an accurate article denied by an agency spokesman because of a headline that went a little far off the mark," Kessler wrote.

"In fact, she diplomatically went out of her way to avoid saying that, without at the same time saying he was qualified. The Washington Post article appropriately noted that she raised questions about his qualifications, but certainly never said or suggested she said Sanders was unqualified," he wrote. "Sanders would have been on safer ground if he had said Clinton is raising questions about his qualifications and now he would like to raise questions about her qualifications. But he can't slam her for words she did not say."