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Find a mistake in your credit report? You’ll want to contact the bureaus, stat.

The bureaus are required to investigate—typically within 30 days—and to fix any problems that are discovered.

The credit bureaus all have a form on their website where you can submit a dispute.

For run of the mill problems, such as a paid-off debt appearing as unpaid, submitting the dispute via the web tool will likely suffice. But if you prefer, you can file a formal letter of dispute via certified mail (you’ll find the address on each credit bureau’s site). The Federal Trade Commission has a sample letter on its website that you can follow. You may also want to contact the creditor—for example, the bank issuing the loan or credit card—to let them know of the error.

Read next: Why Did My Credit Score Drop After I Paid My Credit Card Bill?

For a more serious issue–say, your name has been mixed up with someone else’s – you should definitely file a dispute with both the credit bureau and the creditor(s), as well as the Consumer Finance Protection Bureau (CFPB). The CFPB will follow up with the bureau to make sure your complaint is resolved.