A blunder at a Japanese cryptocurrency exchange let investors briefly buy bitcoins for free - though none were able to profit from the mistake.

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 $US530 million ($680 million) in digital money from Coincheck Inc, with regulators fearing its systems were at risk from cyber-attacks.