Fannie Mae's shifty balance sheet

Rich Duprey says Fannie Mae's assets might not be as valuable as you think:

Fannie's portfolio is vastly overvalued because appraisers routinely overstate the market value of a house by 5%, 10%, 20%, or even more. A portfolio of houses worth less than their appraised values could affect Fannie Mae's ability to sell securities backed by mortgages on those houses. If it's unable to sell its securitized mortgages, the vast real-estate machinery would grind to a halt as the funds juicing the market slow, bringing down the house of cards. Sadly, I think he's right.

It's not only Fannie but other banks that might be overestimating assets.