Tesla won’t allow people to buy its cars using Bitcoin, CEO Elon Musk said this week, citing the cryptocurrency’s high fossil fuel use.

“We are concerned about rapidly increasing use of fossil fuels for Bitcoin mining and transactions, especially coal, which has the worst emissions of any fuel,” Musk said via Twitter on Wednesday. “Cryptocurrency is a good idea on many levels and we believe it has a promising future, but this cannot come at great cost to the environment.”