Elon Musk is no stranger to headlines, drawing eyes and laughs with outlandish tweets and good-natured antics, but tech VC Gene Munster of Loup Ventures says Musk’s latest stunt crossed a line.

“Your behavior is fueling an unhelpful perception of your leadership – thin-skinned and short-tempered,” Loup Ventures’ Munster wrote in an open letter to the Tesla and SpaceX CEO.

Tesla’s stock took a beating Monday on the heels of a quickly-deleted tweet from Musk attacking a Thai rescue diver who assisted with the recovery of 12 stranded boys and their soccer coach from a flooded cave.

“The exchange with [rescue diver] Vern Unsworth crossed the line. I suspect you would agree given you deleted the string from Twitter, but it will take more than that to regain investor confidence,” Munster said in the letter. “Thankfully, the road to regaining investor confidence is well traveled. It starts with an apology. Then, focus your message on your progress toward achieving Tesla’s mission. You might consider taking a Twitter sabbatical.”

Munster wrote the letter “on behalf of investors who believe in you and your mission,” he said, though neither Munster nor Loup Venture own shares of Tesla.

The stock has become something of a confidence index for the CEO, suffering wild swings in recent months as Musk berated analysts on an earnings call and lashed out at journalists on Twitter.

Read Munster’s full letter to Musk.