Fannie Mae is supposed to be prime, but it turns out that much of its loan book is made up of less than perfect credit.

Forbes reports that yesterday “Fannie Mae executives told analysts that 43.0%, or $946 million, of the $2.2 billion in losses incurred during the first quarter involved Alt-A loans. They also said that the company’s ‘Alt-A book will continue to drive an outsize portion of our overall credit losses.’

In much of the discussion about the collapse of the U.S. housing market, commentators have assumed that the massive run-up in property prices that preceded the subprime-mortgage meltdown were simply the result of a speculative frenzy that became a full-fledged market bubble.