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 Feb. 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 said.