Thirteen broker-dealers have been sanctioned by the SEC for violating the rule that protects retail investors in the sale of junk bonds, the SEC announced Monday.

The firms sold bonds issued by Puerto Rico in lots of less than $100,000, which was not allowed, the SEC says. The minimum is set to protect investors who are not capable of absorbing large risks from buying junk bonds, which are riskier than standard municipal bonds. This is the SEC’s first sanction issued under the Municipal Securities Rulemaking Board, which sets the minimum amount of sales in each bond offering.

The SEC Enforcement Division’s Municipal Securities and Public Pensions Unit detected improper sales below a $100,000 minimum denomination set in a $3.5 billion offering of junk bonds by the Commonwealth of Puerto Rico earlier this year, the SEC says.

There were 66 sales below the minimum made by 13 firms, the SEC says. The firms are Charles Schwab & Co., Hapoalim Securities USA, Interactive Brokers LLC, Investment Professionals Inc., J.P. Morgan Securities, Lebenthal & Co., National Securities Corporation, Oppenheimer & Co., Riedl First Securities Co. of Kansas, Stifel Nicolaus & Co., TD Ameritrade, UBS Financial Services, and Wedbush Securities.

Without admitting or denying guilt, the firms agreed to pay penalties ranging from $54,000 to $130,000.

A TD Ameritrade spokesperson says the firm is fully cooperating with the SEC and has agreed to pay the penalty and make any changes needed, without admitting or denying any wrongdoing. Messages left at some other firms were not returned.

LeeAnn G. Gaunt, chief of the SEC’s Municipal Securities and Public Pensions Unit, says, “These firms violated a straightforward investor protection rule that prohibits the sale of muni bonds in increments below a specified minimum. We conduct frequent surveillance of trading in the municipal bond market and will penalize abuses that threaten retail investors.”

Each firm agreed to review their policies and procedures and make any changes that are necessary to ensure proper compliance with regulations, the SEC says.