Roku Inc. (ROKU) - Get Report saw its stock price take a hit Tuesday following a critical tweet by a short-seller raising questions about formidable new competition facing the streaming service.

Roku shares fell 3.1% to $40.88 after Citron Research, in a tweet, called the streaming service's stock "uninvestable now." Roku shares had fallen earlier in the session by as much as 10%.

A key factor in Citron's reassessment was Apple Inc.'s  newly struck deal to begin streaming iTunes movies and TV show on Samsung TV sets.

"We initially went long $ROKU at $35. However, have to recognize when the story has changed," Citron Research wrote in a tweet. "APPLE TEAMING UP WITH SAMSUNG., ROKU CEO selling last week, and short interest at lows. Risk/reward no longer there. Expect big retracement. ROKU stock is uninvestable now.

Roku shares shot up 25% Monday to $42.18 after releasing fourth-quarter results that showed a double-digit jump in active accounts and that topped analysts' expectations.

Roku reported Monday that the number of active accounts topped 27 million in the fourth quarter, a 40% increase from the same period last year.

Meanwhile, the number of streaming hours viewers engaged in jumped 68% year over year to 7.3 billion hours. For the year, the company reported 24 billion streaming hours, a 61% increase from 2017.

A spokesman for Roku declined to comment.

However, Anthony Wood, Roku's chief executive, has previously shrugged off threats of competition from giants like Apple.

Roku benefits from its singular focus on providing a quality streaming service, as opposed to trying to use it as a springboard from which to sell other products, Roku's Wood told Recode Media last year.