component w from 9-axis sensor fusion (MPL) always 0

Hi,

I'm using a mpu9250 and I'm triying to use MPL (MotionDriver) to obtain a 9-axis sensor fusion, but my problem is that the first component resultant from quaternion (w) is always 0.

Using the function 'int inv_get_sensor_type_quat(long *data, int8_t *accuracy, inv_time_t *timestamp)', when I acces to data[0] always is 0, the others 3 components (data[1],data[2] and data[3]) from the quaternion at least they change when I move the chip.

Any idea why is this happening?

Thanks in Advance,

Miguel