CHICAGO (CBS) — In just a few weeks, Cook County homeowners will be getting their property tax bills.

Now imagine you owe nothing.

That happened to a booming business on the South Side.

CBS 2’s Chris Tye reports it’s now part of a large scale investigation.

The former Chicago fire station on West 95th Street now serves as a factory and showroom for Optimo Hatmakers. They produce and sell high end hats online and from a Loop storefront.

But it’s the old firehouse location that recently got the attention of the Cook County Inspector General. Specifically: why in 2017 the property tax bill for the renovated hat factory was zero dollars?

Graham Thompson owns the company. He was working with attorneys to minimize his tax exposure in 2017. But he knew while the old occupants, the city and its firefighters, were tax exempt – he wasn’t.

In a just released report, the Cook County Inspector General “..determined that by erroneously exempting the property and assigning zero assessment value for the tax year 2017, the CCAO effectively granted the taxpayer a tax savings of $10,558.”

The IG said the gaffe was on county assessor who failed to “implement internal controls” to get it right. And moving forward they must “determine the source of the assessment discrepancies.”

It happened on the watch of former Cook County assessor Joe Berrios.

His replacement’s office told CBS 2 about “…changes we’ve made since taking office in 2018 including better internal control and oversight of these types of problems.”

And that “we will continue to improve the operations of our office to ensure all property owners are treated fairly.”

Back taxes will be paid and CBS 2 we uncovered this isn’t the only case where the assessor failed to collect proper taxes.

A home built on an empty lot years ago in Chicago has been paying taxes as an empty lot for years, even as new homeowners have moved in.