Apple sent iPhone 5s reservation codes to customers who didn't make any orders Monday morning, according to a report by local Chinese-language newspaper Sharp Daily.

Those people who received the codes on Monday already bought iPhone 5s devices previously, wondering why they got new codes without making new orders and worrying if their personal data was stolen by hackers, says the media report.

An Apple customer service representative was quoted by the report as saying that the incident was caused by a system error of its outsourcing service provider and those reservation codes sent on Monday can't be used for ordering. The Apple employee added that the incident is under investigation, according to the media report.

iPhone 5s has been available for order in Hong Kong since last month. Buyers must register online and get reservation codes--which Apple sends to their mobile phones--for ordering the new device.