(Bloomberg) -- IRobot Corp. bears picked a bad time to boost bets against the stock.

Shares of the Roomba vacuum cleaner maker surged 16 percent in pre-market trading on Wednesday after the company said international sales growth fueled second-quarter revenue that beat the highest analyst estimate. The company is holding its own against increasing competition and consumers’ preference for its higher-end Roomba models boosted profitability in the quarter, Sidoti & Co. analyst Frank Camma wrote in a research note.

Although the Bedford, Massachusetts-based company outperformed analyst estimates, there is still a robust group of bears betting against it.

More than 45 percent of iRobot’s shares available to trade were on loan to short sellers as of Monday, according to Markit data. That’s close to the 52-week high of 49 percent reached last month and up from 29 percent at the start of the second quarter.

Legal disputes and a potential threat from Amazon.com Inc. may play a role in the short selling. In June, iRobot won the first round in a Roomba patent trade fight over infringement of its patents from other vacuum cleaners including Hoover, bObsweep and iLife. However, Shenzhen ZhiYi’s iLife is seeking a court order declaring it hasn’t infringed any of iRobot’s trademark rights, according to a complaint filed Monday.

The robot manufacturer may also face competition from Amazon which is reported to be working on robots for the home. This could directly compete with iRobot’s popular Roomba vacuum, which has sold more than 20 million units since 2002.

To contact the reporters on this story: Jeran Wittenstein in San Francisco at jwittenstei1@bloomberg.net;Shelly Hagan in New York at shagan9@bloomberg.net

To contact the editors responsible for this story: Catherine Larkin at clarkin4@bloomberg.net, Courtney Dentch

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