ob-ipython freezes

Here are some of the version info

Emacs 25.3 on macOS
org-mode: 20171205
ob-ipython 20180224.953
Jupyter: 4.3.0
IPython: 5.8.0
Python 2.7.14

When I run any code, Emacs freezes and I have to force quit.
#+BEGIN_SRC ipython :session :results raw drawer
a = 1
#+END_SRC

I have been using ob-ipython for about 1 year without problem. I recently updated a bunch of emacs packages. That might have caused some problem.

I think the problem is with the kernel. When I run it with a remote IPython kernel, it works. Is there a way to check what is causing the problem?