IPython sphinx directive fails when promptin isn't in certain formats only for pure python

[MatthewFlamm]

I'm using Sphinx v.1.8.4 and IPython 7.2.0.

For 'regular' .. ipython:: blocks like this,

.. ipython::

   In [1]: print('hello world')

I can use any format for promptin config in my conf.py as long as it has %d in the string, see below for examples I've tried. I'm going to request the requirement for %d be removed, but this is a separate issue.
ipython_promptin = 'In [%d]:'
ipython_promptin = 'In [%d]: '
ipython_promptin = '[%d]'
ipython_promptin = '%d'

However, if I use a pure python block like this

.. ipython:: python

   print('hello world')

I can use the first two custom promptin above, but the last two fail with messages like this:

path/to/python3.6/site-packages/IPython/sphinxext/ipython_directive.py", line 1018, in run
    raise RuntimeError(message)
RuntimeError: Code input with no code at path/to/file.rst