Many people had a good summer — but if you owned stock in Bitcoin Investment Trust GBTC, -1.59% , you’ve had a really, really good summer.

Shares of this publicly traded bitcoin BTCUSD, -1.58% fund jumped $55, or 7%, on Wednesday to close at $830. Your investment would have more than doubled since mid-June, from $382 per share. And the fund is up a staggering 630% since mid-April.

You can stack that up against pretty much any other investment you like and it’s off the charts. There’s no great mystery why this fund is booming. It holds a bunch of bitcoin, the hot cryptocurrency, in its accounts. So when you buy the fund, you are effectively buying into bitcoin.

Read: Confused about bitcoin? 10 things you need to know

The fund allows mom, pop and the dog to speculate on bitcoin in their brokerage accounts — including in their Individual Retirement Accounts. As such it offers a way to buy and sell bitcoin easily, and tax-free. As short-term capital gains, meaning those made within 365 days, are taxed at income tax rates, that can mean a real saving.

Great news, right? Not so fast. If you own shares in the bitcoin trust, you may want to cash it in — sooner rather than later.

Read: The bitcoin party may be coming to an end — this is why

“ The Bitcoin Investment Trust contains a massive, terrifying trap. ”

The Bitcoin Investment Trust contains a massive, terrifying trap — one that many of its investors may not even understand. In a nutshell: You aren’t getting what you think you’re getting.

If you buy the fund for $830 per share, you might imagine you’re getting $830 worth of bitcoin.

Not a chance. You’re not getting anything close to that. Actually, the fund owns less than one-tenth of one bitcoin per share. On Wednesday, with bitcoin itself trading at around $4,600 on crypto exchanges, the currency owned by the trust worked out to just $400 per share.

That’s less than half of the fund’s current share price. So you’re paying $830 for $400 worth of bitcoin — even assuming bitcoin is currently worth its own valuation. What’s supporting the other $430 in the trust’s stock price?

Nothing. Air (and I don’t mean Ether, another cryptocurrency). The bitcoin fund is trading at more than 100% above its net asset value. Buying this fund is the equivalent to paying more than $9,000 for bitcoin when you can buy it on the exchanges for $4,600. Even the ability to avoid short-term capital gains tax isn’t worth a 100% markup.

This price discrepancy is not the fault of the fund’s managers, Grayscale Investments, a crypto-focused asset management firm. It is simply the result of the fund’s popularity. People have rushed to buy it in their brokerage accounts without realizing that they are grossly overpaying. The fund’s price is simply the product of supply and demand. With 1.87 million shares in the fund in issue, the current price values the portfolio at a hefty $1.4 billion. Grayscale declined to comment for this article.

Should you buy Apple stock ahead of the iPhone 8 launch?

What makes this fund even more dangerous is that the Bitcoin Investment Trust is not even a regular stock. Instead it is traded “over the counter,” and there isn’t much liquidity. Even during this bitcoin boom, only about 71,000 shares trade each day.

Think of this as a crowded room with a narrow exit. If lots of people try to leave at once, you’ve got real problems. Meaning, if either bitcoin or the fund crashes, you may have trouble getting out of this investment quickly. And you may certainly have to take a big cut when you sell.

Speculating on bitcoin is one thing. Paying twice the market price is a special kind of stupid.