About 1,800 Kansans who enrolled in health insurance through HealthCare.gov are at risk of losing their coverage if they don’t provide proof of citizenship or immigration status by early next month.

The U.S. Department of Health and Human Services said the 1,800 Kansans were flagged because of inconsistencies in their citizenship or immigration information when they obtained coverage on the federal health insurance marketplace, part of the Affordable Care Act, in late 2013 or early 2014. They must send in proof of their legal status by Sept. 5 or they will lose their coverage Sept. 30.

Illegal immigrants are ineligible for the tax subsidies offered on the marketplace to help with the cost of insurance. More than 57,000 Kansans selected a plan during the marketplace’s first open enrollment period, which ended in April.

HHS is seeking documents from roughly 310,000 Americans who bought coverage on the marketplace. HHS said it has already closed out about 450,000 such cases, but that the remaining people have yet to respond to numerous letters, emails and phone calls.

People who have been contacted by HHS over an inconsistency in their citizenship or immigration status should visit HealthCare.gov, select their application and upload the necessary documents. They can also mail the information (along with the bar code page from the HHS notice) to the London, Ky., address, or call 800-318-2596 for more information.