Elizabeth Warren’s plan to finance “Medicare for All” is a carefully crafted, politically useful document that was written to show how the U.S. could fund a single-payer health system without raising taxes on the middle class. It should give her a little breathing room the next time she’s being grilled by a debate moderator.

It also has some problems.

One, as I wrote Friday, is that its budget assumptions might not be very realistic.

Another, deeper issue—which a number of writers, not to mention Warren’s primary rival Bernie Sanders, have pointed out—is that the centerpiece of her proposal is fundamentally unfair to many businesses and workers. It’s specifically a raw deal for companies that already offer their employees generous insurance, as well as for low-wage workers, who would somewhat indirectly shoulder a disproportionate share of the burden from the tax scheme she has concocted.

To understand the debate over Warren’s plan, it actually helps to start with how Sanders has considered funding single-payer. The Vermont senator has never mapped out a full financing proposal. But he has released a list of options that he would support, including a 7.5 percent payroll tax paid by businesses and a 4 percent income tax on households. In effect, these are really both taxes on workers, since companies tend to pass payroll taxes onto their employees in the form of lower pay.

Hiking taxes on the middle class is not popular. Sanders argues, however, that Americans would save money overall under his plan, since they would no longer have to pay for private insurance or spend out of pocket when they visit the doctor or hospital. But while this would probably be true for most families, for some it would not. Health care spending varies enormously among Americans, and some share of workers would inevitably end up paying more in new taxes under Sanders’ plan than they currently spend on premiums and medical care. The exact proportion of winners to losers would depend on the final tax rates Sanders settled on (his current ones seem a tad low). But there would certainly be middle-class losers. And politically, that’s dangerous.

Warren’s plan is designed to avoid that peril as much as possible. Instead of imposing traditional income and payroll taxes, as Sanders would, she would charge companies a per-worker fee based on what they currently spend on insurance. This “employer Medicare contribution” would initially be equal to 98 percent of a firm’s average health care costs. So, if Starbucks was hypothetically set to spend $20,000 a head on private coverage next year, it would cut the government a $19,600 check for each of its employees. Warren believes that this plan would raise $8.8 trillion over a decade, $200 billion less than what companies are on track to spend now. The plan assures readers that “every company paying for health care today will pay less than they would have if they were still offering their employees comparable private insurance.”

By asking companies to take only the money that they are already expected to spend on health insurance and give it to the Washington instead, Warren makes it difficult to accuse her of hiking taxes on middle-class workers. Strictly speaking, she is imposing a new tax of sorts, but not really increasing their costs. Companies with more than 50 employees are already required to offer them coverage under the Affordable Care Act, and Warren’s plan exempts smaller businesses from having to pay her Medicare fee unless they already provide insurance. Contra Joe Biden, who has accused Warren of pushing a $9 trillion tax hike, she’s only really asking businesses to redirect where they send their cash.

But there are other, much more substantive reasons to dislike Warren’s plan. First, since she would tax companies based on how much they spend on insurance today, her proposal ends up penalizing firms that currently provide their employees more expensive and generous coverage. As the Tax Policy Center’s Howard Gleckman notes, this upside-down reward system is even worse for small businesses, since firms with fewer than 50 employees only have to pay the Medicare fee if they already offer insurance coverage. Those that don’t are off the hook entirely. Presumably, this carve-out is meant to keep mom-and-pop establishments from getting wiped out by new taxes they can’t afford, but it ends up punishing small business owners who offered coverage. Presumably, many of them would not be happy about it.

Over time, some of this unfairness would fade, at least among large companies. Warren’s plan says that the government would gradually adjust each employer’s Medicare contribution up or down to match the national average (small businesses that never offered their workers insurance would still be exempt, as would new firms with fewer the 50 employees, which seems odd). But that leaves another issue: Even when it’s fully phased in, Warren’s plan is still pretty unfair to low-wage workers.

As Matt Bruenig of the People’s Policy Project has explained, Warren’s Medicare fee is basically a “head tax,” meaning that companies pay the same amount for each worker. Starbucks would send Washington $19,600 for its barista and $19,600 for its CEO. What this means is that low-wage workers see a much, much larger share of their potential compensation devoured by health care costs than high earners. It also warps the job market by making low-wage labor relatively more expensive for companies to hire, which makes it harder for some people to find work. With a health care head tax, you’re basically doubling the cost of hiring a dishwasher.

This is, unfortunately, more or less how our current employer-based insurance system works, since private insurance premiums aren’t really connected to what workers earn. Warren isn’t creating new problems so much as replicating ones that already exist. But almost no economist would structure things that way if they were starting a system from scratch. That’s not just me talking by the way; the Berkeley economists Gabriel Zucman and Emmanuel Saez, who have advised Warren on her wealth tax proposals, said the same in their recent book. “No government would out of the blue impose a poll tax to fund health care; it would be a crushing burden on moderate income families,” they write. (A “poll tax” is their very euro term for a head tax.)

Sanders has tried to make hay of this. He’s pointed out that his payroll-tax approach is more progressive, which is true—faced only with a 7.5 percent levy, a Starbucks barista would pay much less for health care under Sanders’ system than the CEO would. And Sanders’ approach isn’t just more economically equitable: It might also make more plain economic sense, since it wouldn’t make low-skill workers vastly more expensive to hire.

But would it make more political sense? Maybe not. On the one hand, Warren’s plan shows that figuring out how to fund single payer is a bit like trying to slay a hydra: You solve one problem, and two more pop up. You trade one set of winners and losers for another. Which means that, maybe, you’re better off just going for the better policy design. On the other, though, Warren can pacify skeptical voters by saying her plan won’t raise taxes on the middle class, which is the whole point of these exercises anyway. It’s not as if anybody is busy writing a plan that’s going to pass Congress.