Zaif, a government-registered exchange run by Osaka-based Tech Bureau Corp, said on Tuesday that a system glitch had let seven customers buy bitcoin with no yen value during a 20-minute window last week.
The exchange voided the trades after discovering the error, which happened on February 16 - though it was still trying to resolve the issue with one customer who tried to transfer the knock-down bitcoins from the exchange, a spokesman told Reuters.
Zaif’s operator had already faced checks after last month’s theft of $530 million in cryptocurrency from Coincheck, with regulators fearing its systems were at risk from cyber-attacks.
Zaif is one of 16 exchanges registered with the government, which last year allowed a further 16 - including Coincheck - to continue operating pending full registration.
It is said that the registered exchanges will form a self-regulatory body from April. They had originally planned to merge two existing industry bodies representing both registered and unregistered exchanges.
The body will set out rules on issues like exchange security and advertising, and will lay out penalties for members who don’t follow the policies, the Nikkei business daily reported on Wednesday.