OpenCV face detection code sometimes raises `'tuple' object has no attribute 'shape'`

I am trying to build a face detection application in python using opencv.
Please see below for my code snippets:

 # Loading the Haar Cascade Classifier
cascadePath = "/home/work/haarcascade_frontalface_default.xml"
faceCascade = cv2.CascadeClassifier(cascadePath)

# Dictionary to store image name & number of face detected in it
num_faces_dict = {}

# Iterate over image directory. 
# Read the image, convert it in grayscale, detect faces using HaarCascade Classifier
# Draw a rectangle on the image    

for img_fname in os.listdir('/home/work/images/caltech_face_dataset/'):
    img_path = '/home/work/images/caltech_face_dataset/' + img_fname
    im = imread(img_path)
    gray = cv2.cvtColor(im, cv2.COLOR_RGB2GRAY)
    faces = faceCascade.detectMultiScale(im)
    print "Number of faces found in-> ", img_fname, " are ", faces.shape[0]
    num_faces_dict[img_fname] = faces.shape[0]
    for (x,y,w,h) in faces:
        cv2.rectangle(im, (x,y), (x+w,y+h), (255,255,255), 3)
    rect_img_path = '/home/work/face_detected/rect_' + img_fname
    cv2.imwrite(rect_img_path,im)

This code works fine for most of the images but for some of them it throws an error -

AttributeError: 'tuple' object has no attribute 'shape'

I get error in the line where I print the number of faces. Any help would be appreciated.