It does not appear on a spreadsheet. It rarely comes up with an advisor. But it sits in the back of the mind of millions of people in their 50s: Eventually, there will be my parents’ house. Eventually, there will be whatever is left in their accounts. It will not solve everything. It will help.

I understand the instinct. I have spent a career building businesses around finance and how long people live, and this assumption turns up everywhere. It is seldom stated out loud. It is almost never stress-tested. And it is getting less reliable every year.

Start with the number everyone has heard. Something close to $124 trillion in American wealth is projected to change hands by 2048, and Gen X is first in line. About $14 trillion of it is expected to reach Gen X households over the next ten years. Set against a generation that saved a fraction of what the boomers had at the same age, that sounds like a rescue arriving.

It is not. Three things get in the way.

The average is a mirage

Averages do real damage in retirement planning, and inheritance is where they do the most.

Only about one in three American households ever receives an inheritance at all. Across all households, Federal Reserve data puts the average received at roughly $46,200. That figure is performing a magic trick. Households in the top one percent average close to $719,000. The bottom half average about $9,700.

The transfer is real. It is also concentrated. The money is not spreading evenly across a generation. It is pooling where wealth already sits. For a median Gen X household, a realistic inheritance is not a retirement plan. It is a good year of saving, if it comes at all.

There is also a gap between what families expect and what shows up. Households that inherit almost always expect more than they receive. The estimate forms early, when parents look healthy and the house is worth what it is worth today. It seldom gets revised downward, even as the years that will consume it pile up.

It arrives too late to do the work

The second problem is timing, and longevity is rewriting it in real time.

The median American who inherits is about 58 years old. Sit with that for a second. The money shows up after the tuition is paid, after most of the mortgage is gone, after the decades when capital could have compounded into something larger.

Inherited money in the hands of a 40-year-old buys a house or starts a business. In the hands of a 60-year-old, it retires a mortgage balance and moves into a conservative portfolio. Same dollars. A different life.

That median age keeps climbing, because parents keep living longer. Longer life is the achievement of our era, not a problem to be solved. But it means the transfer Gen X has half-planned around arrives later every year, at a point in their own lives when it can do less.

Most people set this expectation once, in their forties, and never touch it again. The number in your head is probably a decade old. It formed when your parents were younger, healthier, and far cheaper to care for.

Care gets paid first

Here is the piece that reshapes the whole calculation, and the piece almost nobody has modeled.

Before an estate passes to anyone, it pays for care.

A private room in a nursing home now runs a national median of about $129,575 a year. Assisted living runs about $74,400. Most families assume Medicare covers this. Medicare does not cover custodial care, and custodial care is the bulk of what a long stay involves.

Run it on an ordinary estate. A paid-off house worth $400,000 and $200,000 in savings looks like a meaningful inheritance to a Gen X child doing mental math. Three years of nursing care for one parent takes more than half of it. Add a second parent, or a longer stay, and there is nothing left to pass down.

For most middle-class families, the estate is not a portfolio. It is a house. That matters, because a house cannot be spent in pieces. When care costs land, families sell the home or borrow against it. The asset a Gen X child had mentally earmarked becomes the funding source for a parent’s final years. That is the right use of it. It is also the end of it.

Nobody in that family made a mistake. This is the arithmetic of a long life meeting the price of care in America. The wealth transfer still happens. It transfers to care providers.

What to do instead

None of this is an argument for pessimism. It is an argument for building on ground that will hold.

Take the inheritance out and see whether the plan still stands. Whatever number is sitting in the back of your mind, set it to zero and run the plan again. If it fails, you have found the real gap, and you have found it while there is still time to close it.

Have the conversation now, and make it about care rather than money. Families avoid this because it sounds like asking about the will. It is a different conversation. What is the plan if you need help at 84? Is there coverage for care, and what does it cover? Who manages it when it happens? Families who answer those questions early protect both generations. Families who wait find out during a crisis, at the worst possible price.

Treat whatever arrives as acceleration, not foundation. An inheritance that lands on top of a plan you built yourself is a gift. An inheritance that was holding the plan up, and never comes, is a crisis with no time left to fix it.

Gen X has been handed a hard set of facts. Less saved than the generation before it, no pension underneath, and obligations pointing in both directions at once. The honest response is not to hope the math gets rescued from above.

It is to build something that does not need rescuing.

The opinions expressed in Fortune.com commentary pieces are solely the views of their authors and do not necessarily reflect the opinions and beliefs of Fortune.