The Federal Deposit Insurance Corp has lowered the risk weighting of the bank debt it guarantees to zero from 20 percent — effectively putting the debt on equal footing with risk-free U.S. Treasuries.

That’s where it should be, given the explicit backing of such debt by the government.

Yet Fannie Mae and Freddie Mac debt remains weighted at 20 percent, even though there’s nearly no chance the government would allow the crippled housing giants to default.

The discrepancy illustrates the government’s still wishy-washy backing of Fannie and Freddie. That ambivalence is sure to make Fannie’s and Freddie’s debt more expensive when the Federal Reserve exits the markets next year.

The government has danced around guaranteeing agency debt and mortgage-backed securities for years. Before the crisis, investors let them get away with it. Words like “implicit guarantee” were the “nudge, nudge, wink, wink” of the bond market.

Still, when the Bush Administration put the two into conservatorship, it did everything short of putting the full faith and credit of the United States behind their debt.

There’s good reason it fell short. Such an explicit backing would force the government to put Fannie’s and Freddie’s obligations onto its balance sheet. And their debt alone stands around $1.6 trillion.

But the game of “pretend” is more dangerous to play now than before the crisis. The murky status of the debt appears to be keeping some conservative investors away.

Foreign central banks, for example, have largely abandoned the market. According to Federal Reserve data, their holdings of agency-related debt have shrunk to $761 billion from $986 billion in July last year. Their investment in super-safe Treasuries, meanwhile, has ballooned to $2.1 trillion from $1.4 trillion.

It doesn’t look as if they’re buying the government’s “effective” guarantee that comes in the form of a $400 billion equity line.

So far it hasn’t mattered, because the Fed’s purchases have more than made up the difference. But that program is coming to an end next year.

Even then, there will be enough investors to fill the void, but they’re more likely to want fatter premiums. And that means higher mortgage rates at a time when housing is just starting to stabilize.