It looks like, ndarray.dtype cant be checked using "is" or "is not", it should be compared using == operator:

In [1]: import numpy as np

In [2]: np.array([1,2], dtype=np.ubyte)
Out[2]: array([1, 2], dtype=uint8)

In [3]: x = _

In [4]: x.dtype
Out[4]: dtype('uint8')

In [5]: x.dtype is not np.ubyte
Out[5]: True

In [6]: x.dtype == np.ubyte
Out[6]: True