I am getting "*RuntimeWarning: invalid value encountered in long_scalars*" error in the following code. What is wrong?

>>> a1

[1, 1, 0, 1, 0, 0, 0, 1, 1, 0, 1, 1]

>>> np.sum(a1==1)/np.sum(a1==0)

__main__:1: RuntimeWarning: invalid value encountered in long_scalars