BUFFALO, N.Y. (WIVB)- A typo is costing National Grid big money.

“It was a data entry error which resulted in us billing for assets that actually weren’t in the field in use,” said National Grid spokesperson, Steve Brady.

Those assets are streetlights.

According to an audit, the City of Buffalo overpaid more than $578,000 during a three year period.

There’s 32,000 streetlights in Buffalo; more than 3,500 were over-billed.

According to documents, Buffalo paid on average, about $17,000 extra each month during that three year period.

Street lights aren’t metered; when one is moved or burned out, unless it’s reported to National Grid, they’ll bill for it.

Brady told News 4 this does happen from time-to-time, and the company is working on streamlining their billing process to avoid overcharges in the future.

Back in the 1990s, a Common Council member called for an audit which found a substantial amount of over billing. Since then, it’s something that’s been on the city’s radar.

“You would think in 2016 with the type of technology that we have, that we would be able to do that instantly and be able to look at this on a daily basis, not every 20 years,” City Comptroller Mark Schroeder told News 4.

Buffalo hired Troy and Banks to perform this recent audit. President Tom Ranallo told News 4 utility over-billing is quiet common.

“It happens more than you’d think,” he said.

In fact, the firm is handling claims from a slew of other municipalities in Western New York.

Ranallo said residents are at risk of over-billing too; he said electronic billing makes it easier for small errors to go unnoticed.

“It actually adds to, I think, people not scrutinizing their bills.”

So far, National Grid has paid the city $300,000. They owe another $700,000.

The city has about 15 other claims against the company, that are currently being worked out.