Musk, who has acknowledged he overpaid for Twitter, offered employees new equity grants earlier this year that valued the company at $20-billion. It’s unclear how Fidelity arrived at its new valuation or whether it receives any non-public information from the company.

Fidelity first reduced the value of its Twitter stake in November, to 44% of the purchase price. That was followed by further markdowns in December and February.

Twitter has struggled financially since Musk took over. After saddling the company with $13-billion of debt, Musk’s erratic decision making and challenges with content moderation led advertising revenue to decline by 50%, Musk said in March. An attempt to recoup that revenue by selling Twitter Blue subscriptions has so far failed to take off. At the end of March, less than 1% of Twitter’s monthly users had signed up.

Twitter didn’t specifically respond to a request for comment.

Musk’s investment in Twitter is now worth $8.8-billion, according to the Bloomberg Billionaires’ Index, which uses Fidelity’s valuation to calculate the value of his holding. Musk spent more than $25-billion to acquire an estimated 79% stake in the company last year.

The latest markdown erases about $850-million from Musk’s $187-billion fortune, according to the index. Despite Twitter’s issues, Musk’s wealth is up more than $48-billion this year, largely due to a 63% surge in Tesla’s share price.