The US Securities and Exchange Commission has rejected a proposal from the Winklevoss twins to establish a Bitcoin-based exchange traded fund, and it was clear in its reasoning: the agency can’t be sure that Bitcoin’s price isn’t being manipulated.

Winkle-who wants to create a what? Cameron and Tyler Winklevoss first made a splash in the technology world during the early days of Facebook, as depicted in the 2011 movie The Social Network. They have since become two of the most prominent Bitcoin entrepreneurs. An exchange traded fund, or ETF, is a fund that owns a basket of underlying assets, which could be stocks, bonds, commodities, or other types, and offers tradeable shares to investors. The Winklevoss twins want to create an ETF whose underlying assets are bitcoins.

Strike two: The 3-1 vote to block the proposal is the second time the SEC has rejected an ETF proposed by the Winklevoss twins. The last rejection came in March of 2017. The decision is a setback for Bitcoin enthusiasts, since many observers expect that a regulated Bitcoin ETF would lead to an influx of institutional investors and, in turn, further legitimize the market.

The risk of fraud: In a lengthy analysis (PDF), the SEC laments the lack of data available to study today’s Bitcoin market, where much of the trading goes on in unregulated exchanges in other countries. It cites recent academic research suggesting that certain of those exchanges may be manipulating Bitcoin’s price. And it says that overall it cannot conclude that the Winklevoss’s fund would be sufficiently resistant to fraud.

The beat goes on: The SEC was also clear in its analysis that it has not shut the door on Bitcoin ETFs, and Reuters reports that there are at least five additional ETF proposals still pending. Don’t be surprised if the Winklevoss twins eventually head back to the plate as well.