In most states insurance companies can deny Medigap policies to seniors because of a pre-existing condition, leaving possible long-term care consumers unable to pay for services, a new analysis finds.

Medigap is meant to provide supplemental health insurance for those in traditional Medicare, with one in four beneficiaries holding a policy as of 2015. While Medigap cannot pay for services such as Alzheimer’s care, assisted living or adult day care, it can offer temporary nursing home coverage or hospice benefits.

But the Kaiser Family Foundation analysis found only Connecticut, Maine, Massachusetts, and New York require Medigap insurers to sell policies to all Medicare beneficiaries ages 65 and older, either consistently or for at least a month a year. That means in the majority of the country, seniors may be unaware or incorrect in what protections are offered.

“Medigap is not subject to the same federal guaranteed issue protections that apply to Medicare Advantage and Part D plans, with an annual open enrollment period,” the report states. “ … As older adults age on to Medicare, they would be well-advised to understand the Medigap rules where they live, and the trade-offs involved when making coverage decisions.”