Bitcoin has been on fire in recent years, with the NYSE Bitcoin Index up another 360 percent year-to-date. However, the higher bitcoin prices soar, the more bitcoin bears become convinced the cryptocurrency enthusiasm has created a textbook market bubble.

On Tuesday, one of bitcoin's highest-profile bears reiterated his belief that the Bitcoin Investment Trust (OTC:GBTC) is overvalued by more than 50 percent.

Citron Research’s Andrew Left, who first announced a short position in the GBTC on Sept. 1, provided some insight into his $500 price target for the trust, which currently trades at around $775. According to Left, the GBTC trades at a 70 to 90 percent premium to its net asset value, completely disconnected from the value of its underlying assets.

See Also: 10 Most Ridiculous Cryptocurrencies

Left said the fact that the GBTC couldn’t get its Bitcoin holdings insured at a price of $2,200 per coin means there’s no way they could get them insured anywhere close to the current price of around $4,200 per coin.

“If something is so dangerous that it is uninsurable, do you want to own it?” the new Citron posting reads. “Worse, do you want to own a fund that owns it, while paying a price 70% higher than what the underlying asset is actually worth?”

To prove the point, Citron included a number of popular ETFs along with estimates of their current price premium to NAV. The SPDR S&P 500 ETF Trust (NYSE:SPY) trades at a 0.0015 percent premium to NAV, the SPDR Gold Trust (ETF) (NYSE:GLD) trades at a 0.028 percent premium to NAV and the iShares Silver Trust (ETF) (NYSE:SLV) trades at a 0.01 percent premium to NAV.