A Chinese consumer protection group said Tuesday that a “considerable number” of Apple iPhone users in the country are complaining about their devices spontaneously shutting off. The problem affects iPhone 6 and iPhone 6s units which are reportedly shutting down automatically despite having 50 to 60 per cent of battery power left.

The shutdowns occur in both cold environments and at room temperature, and this persists even after a system upgrade. The phones fail to turn on again until they are plugged in, the association said. Affected iPhone users also took to local social media channels to complain about the issue.

“When the battery is at 60 percent it shuts down,” wrote one user on Sina Weibo. “On restart, the phone will display no battery. Then when I turn it on again, it will be normal, only to automatically shut down again.”

The problem is the latest challenge for Apple in China, where sales are falling as Cupertino faces stiffer competition from homegrown rivals. Apple has invested heavily in China in past few years and it did see outcome in the form of rocketing high sales; however, with Chinese phone makers offering better products, sales in the country have weakened.

Revenue in Greater China, which includes mainland China, Hong Kong and Taiwan, is down 17% in the fiscal year ended Sept. 24, compared to 84% growth in the prior year.

On top of that, the election of Donald Trump may only worsen things. Trump’s describing China as a currency manipulator has stoked concerns about U.S.-China relations with fears of a trade war between the two countries. This could be a set-back for Apple which depends on China for a fifth of its revenue.

Back to the iPhone shutdown issue, the China Consumers Association said it has approached Apple asking what the exact problem is, and how the company plans to address it. Here is the letter the consumer group has sent to Apple asking the firm to reply within 10 days.