US safety officials say you shouldn’t use a self-balancing skateboard (marketed as hoverboards) without a working fire extinguisher nearby.

The Consumer Product Safety Commission is following its British counterparts by investigating several manufacturers, though haven’t yet seized any stock. The warnings come as Amazon decided to offer a full refund to anyone who has bought such a device from it, though it continues to sell some models.

The CSPS says the boards offer two main safety risks. The first is fires, with officials investigating whether the problems are with the lithium-ion batteries themselves or with the circuit boards and safety measures designed to prevent overheating. In the meantime, the agency warns users to “have a working fire extinguisher nearby while charging or using these boards” and to “charge in an open area away from combustible materials.”

The second problem is with the danger of falls. CPSP chairman Elliot Kaye says that although its obvious that such a risk exists, there’s a possibility that some board don’t take enough account of the potential range in weights of users, “potentially leading to the units speeding up or lurching in a manner that a user would not have reason to anticipate, especially a first-time user.” He said the agency is exploring whether the design throws up “a hidden hazard that is leading to fall injuries that should not occur, even on a product that presents some risk of falling. ”

Two standards organizations, ASTM International and UL are both working on specific safety standards for self-balancing boards. UL warns that although some of the components of the board such as batteries and power supplies meet its standards and display labels to that effect, it has not verified the safety of the way these components interact on boards.