More than 60,000 of Verizon D.C.’s residential customers will receive a credit in their August statement to make up for a recent billing error, which is now the subject of an investigation by utility regulators.

Verizon has acknowledged that it started billing in May for a rate increase that wasn’t approved by the D.C. Public Service Commission until June. All told, 60,543 residential customers were overcharged a total of $66,173, Verizon says in filings with the commission.