Face detection not working on specific image

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I am using the haarcascade_frontalface_default.xml classifier to detect faces in python 3 on Windows 7 with OpenCV 3.1.0 using the following code:

import cv2
import sys

classifier = cv2.CascadeClassifier ("haarcascade_frontalface_default.xml")
image = cv2.imread (sys.argv[1])                  # Read image from first command line param
gray = cv2.cvtColor (image, cv2.COLOR_BGR2GRAY)   # Convert the image to grey scale

# Detect faces on image
faces = classifier.detectMultiScale (gray, scaleFactor = 1.1, minNeighbors = 5, minSize = (30, 30), flags = cv2.CASCADE_SCALE_IMAGE)
print ("Found {0} faces!".format (len (faces)))

# Draw a rectangle around the faces
for (x, y, w, h) in faces:
  cv2.rectangle (image, (x, y), (x+w, y+h), (0, 255, 0), 2)

cv2.imshow ("Faces found", image)
cv2.waitKey (0)
cv2.destroyAllWindows ()

This code works fine for most face images I have. However it does not work for this specific image. Any idea why?