numpy masked_where

[Wanderer]

Why does the numpy masked_where create a special case for all False?

import numpy
x = numpy.array([[9,9,9,9,9,9],[9,9,9,9,9,9],[9,9,9,9,9,9],[9,9,9,9,9,9]])
y = numpy.ma.masked_where(x<3,x)
y.mask
Out[1]: False
z= numpy.arange(20)
z.reshape(4,5)
Out[1]: 
array([[ 0,  1,  2,  3,  4],
       [ 5,  6,  7,  8,  9],
       [10, 11, 12, 13, 14],
       [15, 16, 17, 18, 19]])
y = numpy.ma.masked_where(z<3,z)
y.mask
Out[1]: 
array([ True,  True,  True, False, False, False, False, False, False,
       False, False, False, False, False, False, False, False, False,
       False, False], dtype=bool)

Is there a good way to work around this and get an all False array?

Thanks