Exception when converting GIF to RGB and saving

I've reduced the issue down to 2 lines of code:

from PIL import Image
Image.open('plus.gif').convert('RGB').save('plus_thumb.gif', format='GIF')

The image with the issue:

The exception I get:

# python test5.py 
/usr/local/lib/python2.7/site-packages/PIL/Image.py:967: UserWarning: Couldn't allocate palette entry for transparency
  "for transparency")
Traceback (most recent call last):
  File "test5.py", line 8, in <module>
    Image.open('plus.gif').convert('RGB').save('plus_thumb.gif', format='GIF')
  File "/usr/local/lib/python2.7/site-packages/PIL/Image.py", line 1928, in save
    save_handler(self, fp, filename)
  File "/usr/local/lib/python2.7/site-packages/PIL/GifImagePlugin.py", line 465, in _save
    _write_single_frame(im, fp, palette)
  File "/usr/local/lib/python2.7/site-packages/PIL/GifImagePlugin.py", line 383, in _write_single_frame
    _write_local_header(fp, im, (0, 0), flags)
  File "/usr/local/lib/python2.7/site-packages/PIL/GifImagePlugin.py", line 490, in _write_local_header
    transparency = int(transparency)
TypeError: int() argument must be a string or a number, not 'tuple'

This is with Pillow 4.3.0 and Python 2.7.7