Unfortunately, that's an indication of either a disconnected wire (more common) or a faulty potentiometer (less common). The most likely culprit is a loose wire at the potentiometer located in the knee. If she is still under warranty, I would opt for a replacement, but loose wires are fairly easy to fix if she isn't. The repair consists of pealing back the rubber around the foot, rolling up the skin, finding the loose wire (it may still be held in place by glue, but you can carefully peal this back), stripping the end of the wire and soldering it back to the other end of it which should be sticking out of the potentiometer. This problem is pretty common and I've repaired it a few times myself, it is possible that a different issue is causing this though which is why I'd go with a return if the warranty is still good.