DS18B20 Issues - all I get is 'null'

Thanks @DrAzzy. Good to know I've not made any obvious newbie mistakes with something this simple ;)

Code is basic: just the example from the DS18B20 page on the espruino site, reproduced below with my A0 pin in place of A1.

var ow = new OneWire(A0);
var sensor = require("DS18B20").connect(ow);
setInterval(function() {
  console.log(sensor.getTemp());
}, 1000);

I also tried sensor.isPresent() which just returns false.