A major union of federal workers required to work without pay during the shutdown is suing the government, it announced Monday. Some 420,000 employees fall into the category of ‘essential’ workers— border patrol agents, airline security workers and immigration enforcement personnel—who are required to work without pay in shutdowns. After previous government shutdowns, Congress has approved back pay for federal employees, and while that’s likely to happen again, the American Federation of Government Employees, one of the largest federal workforce unions, says it isn’t fair to force people to work without pay in the meantime and disrupt paychecks.