Tesla has delayed the rollout of its Full Self-Driving (FSD) Beta 10.2 software rollout because of last-minute concerns, CEO Elon Musk tweeted early Saturday. He apologized for the delay, saying the release would likely happen Sunday or Monday. He didn’t elaborate on what the “concerns” were.

A few last minute concerns about this build. Release likely on Sunday or Monday. Sorry for the delay.

— Elon Musk (@elonmusk) October 9, 2021

The software was supposed to roll out Friday to about 1,000 Tesla owners with perfect safety scores of 100 —as determined by the automaker— with a gradual rollout for those with scores at 99 and below after that, according to Musk.

Tesla has been testing the beta version of its FSD software for about a year, and despite the head of the National Transportation Safety Board expressing serious concerns about the software’s safety, the carmaker began expanding FSD access to more drivers last month. Before a Tesla driver gets access to FSD, however, the company determines their “safety score,” using criteria that evaluate “the likelihood that your driving could result in a future collision.” The score is tabulated using data collected by sensors in the driver’s Tesla.

The company has required owners who opt in to the FSD beta to sign non-disclosure agreements, and asked that they refrain from sharing video clips online of the system’s mistakes. To reiterate: the FSD software doesn’t make Teslas fully autonomous; Musk himself has said he thinks the “feature complete” version of the software will only be “likely” to drive someone from their home to work without human intervention and will still require supervision.