Tuition reimbursement has long been a benefit of corporate employers. And it’s no wonder. Companies see it as a way to retain current workers, keep employees’ skills up-to-speed, and attract new talent. About five in six—or 83%—of organizations surveyed by the International Foundation of Employee Benefit Plans last year offered some sort of educational assistance or tuition reimbursement to their employees.

While there’s no doubt all employees would appreciate help paying for school, the benefit is especially popular among the fastest growing segment of the workforce—millennials. A 2015 survey by EdAssist, a company that advises employers on their tuition assistance programs, found that if asked to choose between similar jobs, nearly 60% of respondents would pick the job with strong potential for professional development over one with regular pay raises. One in two millennials said they expected an employer’s financial support in paying for further education.

Despite the near-universal appeal of education assistance, just how much help a worker can get from his employer is often limited by—of all things—the United States tax code. The Internal Revenue Service effectively caps the tax-free tuition benefits an employee can receive per year at $5,250—enough for maybe a course or two. Anything over that counts as compensation and the employee incurs taxes accordingly, unless an employee could otherwise count it as a business expense.

The rule makes the unlimited tuition reimbursement offered by the following six employers on Fortune’s Best Companies to Work For list especially rare. Sure, employees at these companies might have to pay taxes on any educational assistance over the IRS’s threshold, but it still beats paying for school all by yourself.