Error: (-5) image is empty or has incorrect depth (!=CV_8U) in function detectAndCompute

Question

I get a problem but I don't understand why it doesn't work. I'm working on MacOSX Sierra and I'm using Python 2.7 and OpenCV 3.1.

It's a very simple script :

import numpy as np
import cv2
#from matplotlib import pyplot as plt

#########################
# SIFT descriptors part #
#########################

img1 = cv2.imread('/Users/valentinjungbluth/Desktop/SIFT:SURF Algo/lampe.jpg',0)
img2 = cv2.imread('/Users/valentinjungbluth/Desktop/SIFT:SURF Algo/ville.jpg',0)

#img1 = cv2.cvtColor(IMG1, cv2.COLOR_BGR2GRAY)
#img2 = cv2.cvtColor(IMG2, cv2.COLOR_BGR2GRAY)

# Initiate SIFT detector
sift = cv2.xfeatures2d.SIFT_create()

print (img1.dtype)

kp1, des1 = sift.detectAndCompute(img1,None)
kp2, des2 = sift.detectAndCompute(img2,None)

bf = cv2.BFMatcher()
matches = bf.knnMatch(des1,des2,k=2)

good = []
for m,n in matches :
    if m.distance < 0.75*n.distance :
        godd.append([m])

img3 = cv2.drawMatchesKnn(img1,kp1,img2,kp2,good,flags=2)

#kp = sift.detect(gray,None)
#img = cv2.drawKeypoints(gray,kp,img)

cv2.imwrite('matches.jpg',img3)

But when I run the script in the terminal, I get exactly this error :

(cv) MacBook-Pro-de-Valentin:SIFT:SURF Algo valentinjungbluth$ python image.py 
uint8
OpenCV Error: Bad argument (image is empty or has incorrect depth (!=CV_8U)) in detectAndCompute, file /Users/valentinjungbluth/opencv_contrib/modules/xfeatures2d/src/sift.cpp, line 770
Traceback (most recent call last):
  File "image.py", line 22, in <module>
    kp2, des2 = sift.detectAndCompute(img2,None)
cv2.error: /Users/valentinjungbluth/opencv_contrib/modules/xfeatures2d/src/sift.cpp:770: error: (-5) image is empty or has incorrect depth (!=CV_8U) in function detectAndCompute

The dtype is uint8 so I don't understand why I get this error. If someone could help me ?

Thank you so much !