OpenCV normalize - histogram remains the same

I am new to OpenCV. I have an 8 bit, grayscale image that I need to normalize in order to improve it's contrast. I'm trying to do so using normalize function, however nothing changes with the image and both histograms are still exactly the same - I thought the histogram should change. Am I doing something wrong? Or I just didn't really understand how normalization should work?

Here is my code:

img = cv2.imread('path_to_file', 0)
normImg = cv2.normalize(img, None, 0, 255, cv2.NORM_MINMAX)
plt.hist(img.ravel(), 256, [0, 256])
plt.show()
plt.hist(normImg.ravel(), 256, [0, 256])
plt.show()
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