That’s true even if they didn’t sign up because they mistakenly thought that having insurance through the ACA marketplaces meant that they didn’t need to enroll in Medicare.

Under a temporary rule change put in place earlier this year, Medicare officials have said they will waive those penalties—but the catch is that the deal ends on September 30.

According to the report, more than 40 groups, including consumer health advocacy organizations and insurers, have already asked Medicare chief Seema Verma to extend the waiver deadline through at least December 31 over concerns that many eligible people are unaware of the waiver’s availability.

In addition, the groups say more time is needed because of application delays at some Social Security Administration (SSA) local offices, where beneficiaries request the waiver.

“We know there are people who can still benefit from it,” Stacy Sanders, the federal policy director at the Medicare Rights Center, a Washington-based advocacy group that coordinated the request to Medicare, says in the report. “We know there have been delays, and those are good reasons to extend it.” Sanders adds, “There’s no reason to expect that people would understand how to move out of the marketplace into Medicare.”

The waiver offer is not just for people over 65 who have kept their marketplace plans, but also for younger people who qualify for Medicare through a disability and chose to use marketplace plans.

It also allows Medicare beneficiaries who earlier realized their mistake in keeping a marketplace plan and have switched to ask for a reduction or elimination of the penalty. But in each case, people had to be eligible for Medicare after April 1, 2013.

According to the report, officials at the Centers for Medicare & Medicaid, which runs Medicare, would not provide details about the number of waivers granted or pending applications and also would not comment on the likelihood of an extension. One applicant for the waiver says in the report that people should know about the waiver before they apply for it, “[b]ecause they don’t seem to want to give you information to help you, you have to know this on your own.”

The report also said that a Social Security Administration spokeswoman declined to comment on whether SSA employees know about the waiver. If the temporary waiver expires, the only other way for beneficiaries to get an exemption is by proving they declined Part B because a government employee misinformed them.

People applying for the waiver, Sanders suggests, should ask Social Security officials for it by using its official name: “time-limited equitable relief.”

There’s another rule most people don’t know about that will affect 2018 marketplace policies, and the groups writing Verma to ask for a deadline extension say that people affected by the second rule will also be helped. This additional rule for the first time will prohibit insurers from issuing a marketplace plan if they know the member is eligible for Medicare and the 2018 policy is significantly different.

Oh, and one last warning: Those who find themselves without a marketplace plan could be in for another surprise: They won’t have insurance for outpatient care until July 1 because Medicare imposes a waiting period before Part B coverage kicks in for latecomers.