Fluke Australia Pty Ltd—Fluke 33X Clamp Meters

A defect has the potential, over time, to cause the meter to display a low or no voltage reading on a circuit energised with a hazardous voltage. In this situation it is imperative that the practice of testing the meter on a known live circuit before and after checking the circuit under test is maintained. Should the meter not pass the "known live circuit test" its use should be discontinued immediately, identified as faulty and removed from service.

More detail here http://www.recalls.gov.au/content/index.phtml/itemId/964924