Amazon is on the defensive over working conditions at a Chinese factory where the company’s Echo speakers and Kindle e-book readers are being made. Workers at the factory were underpaid, had to work long hours and worked under unsafe conditions, according to a China Labor Watch report published this weekend.

The factory in question is being operated by Foxconn, but used primarily to manufacture Amazon devices. China Labor Watch reported that up to 40% of its workforce has been made up of temporary workers, far exceeding a legal limit of 10%. These temp workers received inadequate safety training, weren’t paid extra for overtime and didn’t get paid sick leave, according to the report.

Workers were also subject to excessive hours, at times working 14 days straight, among other violations. “Amazon’s profits have come at the expense of workers who labor in appalling working conditions and have no choice but to work excessive overtime hours to sustain a livelihood,” the nonprofit wrote in its report.

Amazon told The Guardian this weekend that it had audited the factory in question in March, and discovered two violations. “We immediately requested a corrective action plan from Foxconn Hengyang detailing their plan to remediate the issues identified and we are conducting regular assessments to monitor for implementation and compliance with our supplier code of conduct,” the company said.

Amazon’s Echo quickly became the most popular smart speaker after its introduction in 2014. Earlier this year,  Consumer Intelligence Research Partners estimated that the company had sold more than 30 million Echo speakers to date.