Matplotlib/Pandas error using histogram

I have a problem making histograms from pandas series objects and I can't understand why it does not work. The code has worked fine before but now it does not.

Here is a bit of my code (specifically, a pandas series object I'm trying to make a histogram of):

type(dfj2_MARKET1['VSPD2_perc'])

which outputs the result: pandas.core.series.Series

Here's my plotting code:

fig, axes = plt.subplots(1, 7, figsize=(30,4))
axes[0].hist(dfj2_MARKET1['VSPD1_perc'],alpha=0.9, color='blue')
axes[0].grid(True)
axes[0].set_title(MARKET1 + '  5-40 km / h')

Error message:

    AttributeError                            Traceback (most recent call last)
    <ipython-input-75-3810c361db30> in <module>()
      1 fig, axes = plt.subplots(1, 7, figsize=(30,4))
      2 
    ----> 3 axes[1].hist(dfj2_MARKET1['VSPD2_perc'],alpha=0.9, color='blue')
      4 axes[1].grid(True)
      5 axes[1].set_xlabel('Time spent [%]')

    C:\Python27\lib\site-packages\matplotlib\axes.pyc in hist(self, x, bins, range, normed,          weights, cumulative, bottom, histtype, align, orientation, rwidth, log, color, label,    stacked, **kwargs)
   8322             # this will automatically overwrite bins,
   8323             # so that each histogram uses the same bins
-> 8324             m, bins = np.histogram(x[i], bins, weights=w[i], **hist_kwargs)
   8325             m = m.astype(float) # causes problems later if it's an int
   8326             if mlast is None:

    C:\Python27\lib\site-packages\numpy\lib\function_base.pyc in histogram(a, bins, range,     normed, weights, density)
    158         if (mn > mx):
    159             raise AttributeError(
--> 160                 'max must be larger than min in range parameter.')
    161 
    162     if not iterable(bins):

AttributeError: max must be larger than min in range parameter.