ENGINE = opencv_engine with detectors logs "error during face detection"

[daveresbk]

OS:
Ubuntu 16.04

List python packages:
ackports-abc (0.5)
boto (2.38.0)
certifi (2017.11.5)
chardet (2.3.0)
colour (0.1.5)
crcmod (1.7)
derpconf (0.8.1)
futures (3.2.0)
google-compute-engine (2.7.2)
libthumbor (1.3.2)
numpy (1.11.0)
olefile (0.44)
opencv-engine (1.0.1)
pexif (0.15)
Pillow-SIMD (4.3.0.post0)
pip (9.0.1)
pycrypto (2.6.1)
pycurl (7.43.0.1)
Pyste (0.9.10)
pytz (2017.3)
requests (2.9.1)
setuptools (20.7.0)
singledispatch (3.4.0.3)
six (1.11.0)
statsd (3.2.2)
thumbor (6.3.2)
tornado (4.5.2)
urllib3 (1.13.1)
wheel (0.29.0)

In thumbor.conf, I have setup the following detectors:
DETECTORS = [
'thumbor.detectors.face_detector',
'thumbor.detectors.feature_detector',
]

When I use the PIL engine (in this case I use Pillow-SIMD but with PIL the same result), everything works fine:
ENGINE = 'thumbor.engines.pil'

http://<thumbor_server>/unsafe/100x200/smart/https://www.dzoom.org.es/wp-content/uploads/2013/04/retratos-fotograficos-1-734x489.jpg

But when I change to use OPENCV engine, the detectors don't work

http://<thumbor_server>/unsafe/100x200/smart/https://www.dzoom.org.es/wp-content/uploads/2013/04/retratos-fotograficos-1-734x489.jpg

and I see the following in the console output:
2018-01-05 09:41:22 thumbor:WARNING Error during face detection; skipping to next detector
2018-01-05 09:41:22 thumbor:WARNING Error during feature detection; skipping to next detector