Six Chicagoans have sued the Windy City over its new 9 percent tax levied as part of the "Amusement Tax Ruling " that went into effect on September 1.

The tax, which the city of Chicago maintains is "not an expansion of the laws," imposes an additional surcharge on various online services, including Netflix, Spotify, Hulu, Xbox Live, and others.

"We will be adding it to the cost we charge subscribers," Anne Marie Squeo, a Netflix spokeswoman, previously told Ars in a statement. "Jurisdictions around the world, including the US, are trying to figure out ways to tax online services. This is one approach."

One Ars reader, Melissa, told Ars that her monthly Spotify bill had recently increased from $7.99 per month to $8.71 per month. Meanwhile, another Chicago denizen, Freddy Martinez, sent Ars a credit card statement showing that his Spotify bill similarly went from $9.99 to $10.89 monthly as of the beginning of the month.

The lawsuit, which was filed on September 9, alleges that the tax goes beyond the Finance Department’s mandate and violates the federal Internet Tax Freedom Act. If successful, the ruling would be declared invalid and the amount that the plaintiffs had already paid would be refunded.

"What they seem to believe is that the amusement tax language as it existed for years allows them to impose these new taxes on Internet based streaming services, like on Netflix, even though they've never done that before—and I think they're wrong," Jeffrey Schwab, one of the attorneys in the case, told Ars.

"The language in there does not anticipate anything on the Internet. I think what they really want to do is to pass a law that's going to be unpopular, particularly with younger people."

For its part, Chicago is not backing down.

"We are confident that the ruling is a valid application of the existing Amusement Tax," John Holden, a city spokesman, told Ars. "We have every intention of vigorously opposing this lawsuit."

The monthly amount collected is relatively minuscule but can lead to big bucks: the city of Chicago previously told Ars that it expects to collect around $12 million per year as a result.