A new lawsuit claims Uber pays female engineers, and some engineers of color, less than male, white, and Asian-American counterparts.

The complaint (PDF) claims that Uber's biannual "stack ranking" system is invalid, arbitrary, and forces different outcomes between employees "regardless of whether there are meaningful performance differences between individual employees within a particular peer group."

The plaintiffs state that female employees and employees who are African-American, Latino, or American Indian get, on average, lower rankings than male, white, and Asian-American workers, "despite equal or better performance."

Two of the plaintiffs say that they were brought on as independent contractors, rather than employees, resulting in less compensation and less access to company tools and training sessions.

The plaintiffs are Ingrid Avendaño, who worked at Uber as a Software Engineer II from February 2014 to June 2017; Roxana del Toro Lopez, who had the same job title from May 2015 to August 2017; and Ana Medina, who was a Software Engineer I from March 2016 until the present.

It's the latest in a string of lawsuits against tech companies alleging gender bias. Three female engineers sued Google last month . Oracle was sued over gender bias in August. Twitter and Microsoft face similar suits as well, according to Bloomberg

The case, filed Tuesday in San Francisco Superior Court, claims that Uber violated the California Equal Pay Act, as well as the California Private Attorney Generals Act, or PAGA. PAGA claims can theoretically result in large penalties against employers who violate wage laws, but any damages are split, with the state receiving 75 percent of the total and employees receiving 25 percent.

Uber didn't immediately respond to a request for comment about the case.

The lawsuit comes during a difficult year for Uber, which purged 20 employees following an investigation into sexual harassment and a culture of bullying at the ride-hailing startup.