Why is there an extra row of zeros in the histogram of images in a folder?

Question

I have a folder comprising 20 images (.jpg format). I am trying to obtain the histogram of each of the images and store it as a Pandas data frame. My code is shown below:

import os

import cv2
import numpy as np
import pandas as pd

hist = []
for filename in os.listdir("/path/to/images"):
    img = cv2.imread(os.path.join("/path/to/images", filename))
    hist.append(cv2.calcHist([img], [0], None, [256], [0, 256])[:, 0])
histogram = pd.DataFrame(hist)

I expect the data frame "histogram" to be of shape (256,20). However, I get a (256,21) shaped data frame with one row full of zeros. I have no idea why this row of zeros appears. Is there an error in my code?