Many Americans buying insurance coverage for 2014 may never get the chance to claim new federal tax credits to subsidize the cost of health insurance, due to an odd wrinkle in the signup process.

The problem will impact individuals who currently receive insurance on the individual market, but only if they fail to take action in the coming months. Hundreds of thousands—if not millions—of people who purchase coverage independently are now receiving letters from insurers canceling policies that do not comply with new Affordable Care Act (ACA) regulations. In cancelling such plans, some insurers are telling customers they will be automatically enrolled in alternative ACA-compliant coverage unless they object.

This could be a major snag in the ACA’s plan to subsidize insurance purchased on the individual market. New tax credits, available to individuals earning less than 400 percent of the federal poverty level, or about $46,000 per year, can only be accessed through new ACA insurance marketplaces. Those who purchase coverage outside the exchanges cannot claim subsidies, even if they qualify for them, according to the Centers for Medicare and Medicaid Services, the agency overseeing implementation of the ACA. Automatic enrollment directly through an insurer would avoid the exchanges, and the subsidies, entirely.