Though Medicare is an essential part of retirement, pre-retirees continue to have misconceptions on how the federal program works.

Close to 90 percent of older Americans either enrolled in Medicare or plan to sign up for coverage, according to a recent survey from Nationwide Retirement Institute.

The insurer’s research arm worked with The Harris Poll to survey 1,007 adults over age 50 with a household income of at least $150,000.

More than 7 out of 10 participants said that they wish they better understood Medicare coverage.

“Health care costs are the biggest expense in retirement and should be a major factor when estimating retirement expenses, since they are often costly and unexpected,” said John Carter, president of retirement plans for Nationwide.

“It’s an opportunity for financial advisors to educate their clients and for consumers to ask questions,” he said.

Here are three major facts that even affluent pre-retirees are getting wrong about Medicare.