The U.S. Equal Employment Opportunity Commission is alleging that the nation’s largest private employer violated the Americans with Disabilities Act by firing a longtime employee with Down syndrome.

Marlo Spaeth worked at a Wisconsin Walmart store for 15 years, receiving multiple pay raises and satisfactory performance reviews, when managers changed her schedule, according to a lawsuit filed this week in the U.S. District Court for the Eastern District of Wisconsin.

Spaeth, who was accustomed to working the hours of noon to 4 p.m., was expected to work later hours and longer shifts assigned by the retailer’s new computerized scheduling system. However, because of her Down syndrome, she struggled with the change in routine and was disciplined for absenteeism and ultimately terminated, the complaint said.

After she was fired, Spaeth’s sister and guardian, Amy Stevenson, stepped in and asked Walmart to rehire Spaeth and allow her to work her regular afternoon shift as a reasonable accommodation, but Walmart refused, the suit alleges.

“Walmart’s refusal to accommodate such a simple scheduling request here is not only a violation of the ADA, it’s also an example of how easily a successful 15-year career can be harmed by disability discrimination,” said Gregory Gochanour, a regional attorney for the EEOC’s Chicago District.

The federal commission said it tried unsuccessfully to reach a settlement with Walmart before going to court. The lawsuit asks the court to order Walmart to rehire Spaeth with back pay and compensatory and punitive damages.

In a statement, Ragan Dickens, director of national media relations for Walmart, said the company’s expectations for Spaeth were clear.

“When a store’s needs change, we often adjust associate schedules to meet those demands. Ms. Spaeth’s job requirements were no different. We repeatedly spoke with Ms. Spaeth about the importance of working her full work schedule and she told us she understood. Despite this, she continually failed to complete her assigned shift and that is the reason she is no longer with the company,” Dickens said. “We’re sensitive to this situation and we tried to reach an amicable resolution that would support Ms. Spaeth. We remain open to continuing those discussions, but the EEOC has not acted in her best interest.”