Python 2.7.10, matplotlib 1.5.1, GNU/linux
Python from Ubuntu; matplotlib from pip --user

import pylab as plt
import numpy as np
x=np.arange(10,20)
y=np.arange(10,20)
plt.arrow(x,y,x+1,y*0)

Line 1170 in patches.py (if not length) fails in above if the distance+head_length is a numpy array rather than a list:
ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()