bins must increase monotonically

I just want to draw Matplotlib histograms from skimage.exposure but I get a ValueError: bins must increase monotonically. The original image comes from here and here my code:

from skimage import io, exposure
import matplotlib.pyplot as plt

img = io.imread('img/coins_black_small.jpg', as_grey=True)
hist,bins=exposure.histogram(img)

plt.hist(bins,hist)

ValueError: bins must increase monotonically.

But the same error arises when I sort the bins values:

import numpy as np
sorted_bins = np.sort(bins)
plt.hist(sorted_bins,hist)

ValueError: bins must increase monotonically.

I finally tried to check the bins values, but they seem sorted in my opinion (any advice for this kind of test would appreciated also):

if any(bins[:-1] >= bins[1:]):
    print "bim"

No output from this.

Any suggestion on what happens?
I'm trying to learn Python, so be indulgent please. Here my install (on Linux Mint):

• Python 2.7.13 :: Anaconda 4.3.1 (64-bit)
• Jupyter 4.2.1