def METHOD_NAME(self):
    """
    Check it correctly fails if the device with the given serial number is
    not a MFF.
    """
    # Look for a device with a serial number not starting with 37
    paths = glob.glob("/sys/bus/usb/devices/*/serial")
    for p in paths:
        try:
            f = open(p)
            snw = f.read().strip()
        except IOError:
            logging.debug("Failed to read %s, skipping device", p)
        if not snw.startswith("37"):
            break
    else:
        self.skipTest("Failed to find any USB device with a serial number")
    kwargsw = dict(KWARGS)
    kwargsw["sn"] = snw
    with self.assertRaises(ValueError):
        dev = CLASS(**kwargsw)