SAN FRANCISCO (KGO) — Wednesday marked a bump in the road for Amazon’s self-driving unit, Zoox.

The company says it recalled software on over 250 vehicles due to issues with its automated driving system.

MORE: Zoox robotaxis begin testing on San Francisco streets

According to Zoox’s report filed with the National Highway Traffic Safety Administration, certain versions of its automated system could cause unexpected hard braking.

“The recalled software is no longer in use, and no Zoox vehicles on the road operate with this software today,” said Zoox’s Scott Coriell in an email to ABC7 News.

The Foster City-based company has not publicly disclosed the size of its fleet, but it has increased testing over the past year.