The glass inside Apple Park in Cupertino, California, is now a little less transparent.

Apple placed safety decal stickers onto the glass inside its headquarters to prevent employees from walking into glass panels and hurting themselves, the USGlass News Network reports.

In early January, employees began to move into Apple’s giant spaceship-like campus covered in glass, and weeks later reports surfaced that a handful of employees walked into glass and hurt themselves. In some cases 911 was called and paramedics treated minor injuries.

The collisions occurred not long after employees moved into the new $5B Silicon Valley headquarters. Many did so begrudgingly, complaining about the open-office concept and pushing back against the lack of private space, cubicles and closed offices.

California’s Division of Occupational Safety and Health sent Apple a letter on Jan. 25  informing the company that it needed to mitigate the situation. Apple then put dashed-shaped decal stickers on the glass at eye level. The agency closed its inspection via letter on March 22.

California Department of Industrial Relations Public Information Officer Paola Laverde told USGNN that the January incidents did not cause severe injury to any employees that would have required a report from Apple.

Apple’s 2.8M SF $5B campus has over 3,000 curved glass panes in its main building.