Why doesn't this OpenCV masking function iterate?

I wrote an algorithm in Python OpenCV to find certain targets, but sometimes these targets are hard to find so I made this if-else statement to just output 'target not found' when it fails to find the target. I'm iterating over 1000 images and calling the algo on them, but i get this error:

'NoneType' object is not iterable

on 6th line of the code below:

def image_data(img):
    img3 = masking (img)
    if img3 is None:
        print "target not found"
    else:
        cent, MOI = find_center(img3)
        if cent == 0 or MOI == 0:
        print 'target not found'
        else:
        return cent[0],cent[1],MOI

I understand it means it did't find the image, but why doesn't it just go on to the next image and print error statement?