OpenCV bitwise_and

Question

I'm very newbie in the OpenCV programming. I'm learning about the image bitwise operation. I'm following this example http://docs.opencv.org/trunk/d0/d86/tutorial_py_image_arithmetics.html, but I had a very strange result.
These are the input images:

This is the python code:

# Load two images
img1 = cv2.imread('messi5.jpg')
img2 = cv2.imread('opencv-logo.png')
# I want to put logo on top-left corner, So I create a ROI
rows,cols,channels = img2.shape
roi = img1[0:rows, 0:cols ]
# Now create a mask of logo and create its inverse mask also
img2gray = cv2.cvtColor(img2,cv2.COLOR_BGR2GRAY)
ret, mask = cv2.threshold(img2gray, 10, 255, cv2.THRESH_BINARY)
mask_inv = cv2.bitwise_not(mask)
# Now black-out the area of logo in ROI
img1_bg = cv2.bitwise_and(roi,roi,mask = mask_inv)
# Take only region of logo from logo image.
img2_fg = cv2.bitwise_and(img2,img2,mask = mask)
# Put logo in ROI and modify the main image
dst = cv2.add(img1_bg,img2_fg)
img1[0:rows, 0:cols ] = dst
cv2.imshow('res',img1)
cv2.waitKey(0)
cv2.destroyAllWindows()

This is the expected result:

And this is the result of the code on my PC:

How is it possible? Same pitcure, same code and so different result. Thank you to all

These are the intermediate image:

Answer 1

in the tutorial a different image for the logo is used: it has a black background and white letters - as you can see in the first figure on the page you cited :)