Shares of Fannie Mae fell sharply today upon a report published by Fortune Thursday that questioned the GSE’s method of accounting for credit losses. From the original story:

Last week, as part of its earnings report, Fannie Mae revealed that the company had changed the way it calculates the credit loss ratio. Under the new method, Fannie Mae’s annualized credit loss ratio was just 4 basis points in the first nine months of the year … Under the previous method, Fannie Mae would have been well outside of its range. The company would have reported an annualized loss ratio of 7.5 basis points in the first nine months of this year.

Fannie responded quickly to the allegations that it might be playing a shell game with its credit risk, defending the change as part of an effort to make the numbers more transparent. The GSE held a conference call today, which the Associated Press reports was apparently a testy one:

Analysts peppered the executives with questions in a skeptical tone … “This just smacks too much of the accounting games the company was playing a couple years ago,” said Armando Falcon, who headed the Office of Federal Housing Enterprise Oversight, the federal agency regulating Fannie at the time its accounting crisis erupted. “They have very little room to play with here when it comes to trust and credibility,” Falcon said in a telephone interview. He said the executives failed to explain why the change was made at this particular time.

Fannie’s shares are still off roughly 5.5 percent in late trading as of when this post was published, after rebounding somewhat from a ten percent fall earlier in the day.