Oppenheimer has agreed to pay Michigan regulators $900,000 for failing to register 40 of its investment adviser representatives as required by state law.

Oppenheimer, which admits no wrongdoing in agreeing to pay the fines, will also send a letter of explanation to the clients who had business with the 40 advisers at some point between Oct. 1, 2009 and the end of 2015.

Oppenheimer admitted that it made a mistake when it did not register 40 advisers with the state of Michigan.

Though Oppenheimer doesn’t come out with an outright apology in the letter, it does admit that “we made a mistake involving our Michigan representatives that has since been corrected.” The letter explains that “regrettably we did not catch it” when the state started requiring the registration and licensing of investment adviser representatives in 2009.

“Upon discovering our error in 2015, we registered a number of our financial advisers as investment adviser representatives who were previously only registered as our broker-dealer agents,” the letter states. “Except for that registration mistake, the investment adviser representative was otherwise qualified, was registered in other states, and had passed all necessary exams and other requirements. Again, this was a technical error that we do not believe affected the quality of services you were provided, the advice you were given, or the validity of any of your business.”

Michigan’s Corporations, Securities & Commercial Licensing Bureau launched its investigation of Oppenheimer after noticing the firm’s unusually large volume of adviser registrations in late 2015.

The $900,000 to be paid to the bureau includes a $250,000 civil fine for failing to register the 40 advisers, a $400,000 fine for their unregistered work in the state and five annual payments of $50,000 to the bureau's Securities Investor Education and Training Fund. Oppenheimer has 12 offices in Michigan.

Oppenheimer has suffered a few regulatory black eyes in 2015 and 2016. In June, Oppenheimer agreed to pay $2.25 million in fines to FINRA and $716,000 in restitution for failing to implement policies that would have prevented the firm from soliciting retail customers for nontraditional ETFs.

Read more: FINRA sanctions Oppenheimer $2.9M for unsuitable sales of nontraditional ETFs

And in January 2015, Oppenheimer agreed to pay $20 million to settle related cases by the SEC and the Department of the Treasury's Financial Crimes Enforcement Network about its failure to report that its offshore client Gibraltar Global Securities was improperly selling billions of shares of penny stocks.

Read more: Oppenheimer to Pay $20M Over Penny-Stock Sale Violations

Oppenheimer did not respond to a request for comment.