Dear Liz: Someone recently asked you if signing up for Medicare is mandatory. Your answer implied no, one does not have to sign up at 65. However, it is my understanding that if a person does not enroll when first eligible, they will be hit with large penalties on their Medicare premiums if they sign up later. Am I missing something?

Answer: Not at all. That answer was too short and should have mentioned the potentially large, permanent penalties most people face if they fail to sign up for Medicare Part B and Part D on time.

To review: Medicare is the government-run healthcare system for people 65 and older. Part A, which covers hospital care, is free. Medicare Part B, which covers doctor’s visits, and Part D, which covers prescriptions, typically require people to pay premiums. Many people also buy Medigap policies to cover what Medicare doesn’t, or opt for Medicare Part C. Part C, also known as Medicare Advantage, is an all-in-one option that includes everything covered by Part A and Part B and may include other benefits.

There’s a seven-month initial enrollment period that includes the month you turn 65 as well as the three months before and three months after.

People who don’t sign up when they’re first eligible for Part B usually face a penalty that increases their monthly cost by 10% of the standard premium for each full 12-month period they delay. For Part D, the penalty is 1% of the “national base beneficiary premium” ($33.19 in 2019) times the number of full months the person was uncovered.

People who fail to enroll on time also could be stuck without insurance for several months because they may have to wait until the general enrollment period (Jan. 1 to March 31) to enroll.

People typically can avoid these penalties if they have qualifying healthcare coverage through a union or an employer (their own or a spouse’s). When that coverage ends, though, they must sign up within eight months or face the penalties. Also, they might not avoid the penalties if their employer-provided coverage becomes secondary to Medicare at 65, which can happen if the company employs fewer than 20 workers. Anyone counting on union or employer coverage to avoid penalties should check with the company’s human resources department and with Medicare to make sure they’re covered.

The original letter writer had no income to pay Medicare premiums, so the answer also should have included the information that Medicaid — the government healthcare program for the poor — might help pay the premiums. People in this situation should contact the Medicaid office in their state. (Medicaid is known as Medi-Cal in California.)

Liz Weston, Certified Financial Planner, is a personal finance columnist for NerdWallet. Questions may be sent to her at 3940 Laurel Canyon, No. 238, Studio City, CA 91604, or by using the “Contact” form at asklizweston.com.