import matplotlib.pyplot as plt
import numpy as np
plt.plot(np.sin(np.linspace(-5, 5, 100)), 'v', c='none')
plt.savefig("asdf.pdf")

TypeError: 'NoneType' object is not subscriptable

This happened to me because I was using matplotlib 1.5 code, where this was actually not invisible.
Changing markeredgecolor='k' (which was the default before) fixes the problem. But that is a pretty surprising error and might hint at something going wrong somewhere?

In particular, png rendering works fine, only pdf errors.