Joe Knollenberg’s calculator must need new batteries.

The Oakland County congressman is in hot water in the House for dramatically underreporting the value of his Washington, D.C., home on his personal financial disclosure forms, according to a front-page story in Roll Call on Tuesday.

On the Republican’s most recent disclosure forms Knollenberg reported the value of his property at $50,000 to $100,000.

Its actual assessed valued? $781,840.

 What’s more, his reporting of the home’s value has dropped, from the $100,000 to $250,000 range he reported last year.

When a property provides rental income — as Knollenberg’s home does — House rules require members to report the entire property’s gross value on financial disclosure forms, even if only part of the property is rented out.

Details about House members’ mortgages have come under increased scrutiny since the subprime mortgage disaster. If Knollenberg knowingly underreported the value of his home, it may be because he did’nt want to publicize the fact that his second home is worth more than three-quarters of a million dollars.

Knollenberg’s spokesman told the Capitol Hill newspaper that the inaccurate report was a “clerical error” and an “inadvertent mistake [that] will be corrected immediately.”

Michigan Democratic Party Chair Mark Brewer had a different take. In a press release he called Knollenberg’s report “just another example of the broken ethics that pass for normal behavior inside the Beltway.”