thumbor: too many error messages on thumbor webp

2014-07-29 17:28:24 thumbor:ERROR Image format not found in PIL: .webp
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/thumbor/engines/pil.py", line 136, in read
    self.image.save(img_buffer, FORMATS[ext], **options)
  File "/usr/local/lib/python2.7/dist-packages/PIL/Image.py", line 1459, in save
    save_handler = SAVE[format.upper()] # unknown format
KeyError: 'WEBP'
2014-07-29 17:28:24 thumbor:ERROR Image format not found in PIL: .webp
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/thumbor/engines/pil.py", line 136, in read
    self.image.save(img_buffer, FORMATS[ext], **options)
  File "/usr/local/lib/python2.7/dist-packages/PIL/Image.py", line 1459, in save
    save_handler = SAVE[format.upper()] # unknown format
KeyError: 'WEBP'
2014-07-29 17:28:25 thumbor:ERROR Image format not found in PIL: .webp
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/thumbor/engines/pil.py", line 136, in read
    self.image.save(img_buffer, FORMATS[ext], **options)
  File "/usr/local/lib/python2.7/dist-packages/PIL/Image.py", line 1459, in save
    save_handler = SAVE[format.upper()] # unknown format
KeyError: 'WEBP'

# pip list
argparse (1.2.1)
backports.ssl-match-hostname (3.4.0.2)
boto (2.3.0)
Cheetah (2.4.4)
cloud-init (0.7.2)
configobj (4.7.2)
derpconf (0.6.0)
GnuPGInterface (0.3.2)
oauth (1.0.1)
Pillow (2.3.1)
pip (1.5.6)
prettytable (0.6.1)
pycrypto (2.6.1)
pycurl (7.19.0)
python-apt (0.8.8.2)
python-magic (0.4.6)
PyYAML (3.10)
requests (0.12.1)
setuptools (5.4.1)
six (1.1.0)
thumbor (4.2.1)
thumbor-pexif (0.14)
tornado (3.2.2)
unattended-upgrades (0.1)
wsgiref (0.1.2)