Shares of Fannie Mae and Freddie Mac have been riding a wave of optimism in the last week. Several analysts have estimated that the mortgage giants have sufficient capital for the year, and a management shakeup at Fannie has helped bolster investors' confidence.

Indeed, since last week, shares of Fannie have risen 60 percent.

Yet while stock investors in the United States may be more optimistic, some important debt investors are not.

The Bank of China, one of China's four big commercial banks, has cut its holdings of Fannie and Freddie securities by more than a quarter, from nearly $11 billion at the end of June, report Saskia Scholtes  and James Politi  of the Financial Times.

It's just one institution, to be sure, but it is a dangerous sign. Foreign institutions, in particular Asian central banks and state-run banks like the Bank of China, have been the main customers for Fannie and Freddie debt and securities. That flood of foreign cash helped lubricate the boom in housing and credit that has since become a bust.

Even amid the credit storm of the last year, foreign investors have largely hung on to Fannie and Freddie paper. But fears of a Treasury Department bailout of the two government-sponsored corporations may have proved too ominous to keep the faith. Bloomberg News has noted that Japanese investors have been big sellers of Fannie and Freddie securities.

New blood and the hope of new capital are apparently not enough to stop a growing squeamish in Beijing, Tokyo, Singapore, and elsewhere over U.S. mortgage debt.  And that will make Fannie's and Freddie's efforts to roll over some $223 billion of securities by the end of next month that much more difficult.

Citing Federal Reserve data, the Financial Times says that it appears that foreign investors are buying Treasury securities at the same time they are trimming their holdings of Fannie, Freddie, and government agency debt.

The Calculated Risk blog speculates that this selling may explain "why the spread between Fannie and Freddie debt yields and Treasury debt is so high."

Or perhaps investors have just seen this video.