Delta Air Lines and United Airlines are facing proposed class-action lawsuits from passengers who claim they paid extra for window seats that had no actual windows, Business Insider reports.

Filed Tuesday, the lawsuits allege that each airline “has likely sold over a million windowless ‘window’ seats” to unsuspecting travelers. Delta customers reportedly spent more than $70 to select a preferred seat, including $40 for upgrades to higher ticket tiers, according to the complaint. For United, passengers often paid over $50 for domestic flights and up to $100 for international flights.

The suits highlight a design issue affecting certain aircraft. On Delta’s Boeing 757s, as well as Boeing 737s and Airbus A321s used by both airlines, some seats are positioned next to blank walls rather than windows due to the placement of air conditioning ducts, electrical conduits, and other internal components. On a Boeing 737-800, for instance, the seats affected are typically 10A, 11A, or 12A, depending on the plane’s configuration.

Passengers argue that paying extra for what was advertised as a window seat constitutes a misleading practice and are seeking compensation through the class-action suits.