Investigators have found that 99 percent of fake Apple chargers sold online failed a basic safety test, the BBC reports. Safety specialists UL tested 400 counterfeit chargers bought from 8 countries—including the U.S., China and Australia—and found that only three had enough insulation to guard against electric shocks. Trading Standards, the group that commissioned the study, warned buyers to be wary of chargers that don’t seem to fit easily into a socket or don’t bear a manufacturer’s logo and safety certification mark. They also warned that some safety markings are being faked, recommending that buyers check to see if their charger comes with instructions that include conditions for proper use, basic electrical safety advice, and details about how to dispose of the device safely. Apple is currently suing one third-party vendor that the company said put customers “at risk” with its counterfeit power adapters on Amazon.