Elon Musk, who has become a force in the cryptocurrency universe, said Tesla is no longer accepting Bitcoin as payment for purchases of its electric vehicles owing to the excessive amount of carbon-based energy it uses. Bitcoin fell more than 6% following his tweet.

“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 tweeted on Wednesday afternoon. “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.”

While Tesla isn’t going to accept Bitcoin as payment, the company’s billionaire CEO said it also won’t be selling holdings in the cryptocurrency and still plans to use “it for transactions as soon as mining transitions to more sustainable energy.”

The move is a curious one since Tesla’s recent $1.5 billion Bitcoin investment triggered a surge in the currency’s value. It also helped the carmaker report a first-quarter project this year, owing to the $101 million gain it reported from selling 10% of its Bitcoin holdings. He’s also made comments that pumped up and then undercut Dogecoin, a cryptocurrency that started as a joke.

Musk said the company is looking at other cryptocurrencies that “use “<1% of Bitcoin’s energy/transaction,” without identifying specific alternatives. 

Bitcoin dropped 6.2% to $52,960 at 7:59 p.m. New York time on Wednesday, while Tesla shares fell 4.4% to $589.89 in Nasdaq trading.