floating point - Why does scikit-image return NaN for the color difference of this color with itself? -

i have following snippet:

import numpy np skimage import color  c = np.array([0., 1.59607713, 0.87755709]) print(color.deltae_cmc(c, c)) 

i expected code print 0 or number close 0, prints nan instead. why? knowledge c valid color in l*a*b* color space.