Investors should take profits after Roku shares more than doubled in three days, according to one Wall Street firm.

Oppenheimer on Tuesday lowered its rating to underperform from perform for Roku shares, citing the company’s excessive valuation.

“While Roku has established itself as the leading independent OTT streaming platform though a device and software strategy, the stock is now the most expensive publicly traded Internet-based company, on the basis of Platform revenue or Platform gross profit,” analyst Jason Helfstein wrote in a note to clients. “In our view, the stock is trading on non-fundamental factors, driven by a limited float … and high short interest.”

Roku shares declined 6 percent Tuesday following the report.

Helfstein started his Roku price target at $28, representing 34 percent downside to Monday’s close.

The analyst noted the company is trading at 18 times fiscal 2018 estimated gross profit versus the 4 times average of its technology industry peers.

It is “difficult to justify this valuation, even with secular growth trajectory and market position,” he wrote.

Roku shares have been extremely volatile since its IPO. The stock rallied nearly 70 percent on its first day of trading and then declined more than 20 percent in less than month.

The roller coaster ride continued as the company’s stock rose more than 120 percent in the three trading days after it reported better-than-expected earnings Wednesday.

Roku declined to comment for this story.

— CNBC’s Michael Bloom contributed to this story.