Invalid base64 data

Hi thanks for this awesome package first.

I've been using ob-ipython for a while, it worked like a charm until recently. Try to run a simple inline chart

#+BEGIN_SRC ipython :session :exports both :results raw drawer
%matplotlib inline
import matplotlib.pyplot as plt
import numpy as np
plt.hist(np.random.randn(20000), bins=200)
#+END_SRC

However the system gave me the Invalid base64 data error, ob-ipython--write-base64-string: Invalid base64 data. Any idea about the issue?

About my environment: emacs 25.3.2; ipython 6.2.1; jupyter 4.3.0 (both jupyter_console and jupyter_client are installed)

Thank you.