TypeError: ufunc 'isfinite' not supported for the input - in matplotlib stackplot

The following code, which calls stackplot twice, works on the first but fails on the second. On the call to the second the int list looks exactly the same.

Numpy 1.10.4 Matplotlib 1.5.1 Python 3.5.1

import sys
import numpy as np
import pandas as pd
import random
import matplotlib.pyplot as plt

plt.ion()
period = 36
ward_dates = pd.date_range(start=pd.datetime.today(), freq='M', periods=period)
list_size = 6
ward_counts = [[] for i in range(list_size)]
for idx in range(list_size):
    current_list = [np.random.randint(0, 500000) for r in range(period)]
    ward_counts[idx] = current_list
plt.stackplot(ward_dates, ward_counts)
plt.close('all')

units = np.random.choice(['U1', 'U2', 'U3'], size=period)
index = pd.MultiIndex.from_arrays([units, ward_dates])
s1 = pd.Series([np.random.randint(0, 5000) for r in range(period)], index=index)

ward_counts = [[] for i in range(3)]
l1 = [int(x) for x in s1.loc['U1'].values]
ward_counts[0] = l1
l1 = [int(x) for x in s1.loc['U2'].values]
ward_counts[1] = l1
l1 = [int(x) for x in s1.loc['U3'].values]
ward_counts[2] = l1
plt.stackplot(ward_dates, ward_counts)`

Error trackback is

Traceback (most recent call last):
  File "<input>", line 29, in <module>
  File "C:\Users\dgrawroc\Documents\Anaconda3\envs\m2\lib\site-packages\matplotlib\pyplot.py", line 3326, in stackplot
    ret = ax.stackplot(x, *args, **kwargs)
  File "C:\Users\dgrawroc\Documents\Anaconda3\envs\m2\lib\site-packages\matplotlib\__init__.py", line 1811, in inner
    return func(ax, *args, **kwargs)
  File "C:\Users\dgrawroc\Documents\Anaconda3\envs\m2\lib\site-packages\matplotlib\axes\_axes.py", line 4443, in stackplot
    return mstack.stackplot(self, x, *args, **kwargs)
  File "C:\Users\dgrawroc\Documents\Anaconda3\envs\m2\lib\site-packages\matplotlib\stackplot.py", line 113, in stackplot
    **kwargs))
  File "C:\Users\dgrawroc\Documents\Anaconda3\envs\m2\lib\site-packages\matplotlib\__init__.py", line 1811, in inner
    return func(ax, *args, **kwargs)
  File "C:\Users\dgrawroc\Documents\Anaconda3\envs\m2\lib\site-packages\matplotlib\axes\_axes.py", line 4608, in fill_between
    y2 = ma.masked_invalid(self.convert_yunits(y2))
  File "C:\Users\dgrawroc\Documents\Anaconda3\envs\m2\lib\site-packages\numpy\ma\core.py", line 2293, in masked_invalid
    condition = ~(np.isfinite(a))
TypeError: ufunc 'isfinite' not supported for the input types, and the inputs could not be safely coerced to any supported types according to the casting rule ''safe''

[Nocturnehub]

when I run this part of my code, this error appeared ,I don’t know whats the problem? when i run it for year 2019 that exist in my data set, the figure was shown ,but it doesn’t show the prediction year’2020’

[seberg]

Numerical types should go through fine, but object type for example should fail like this. So Probably got an object array somewhere, could also be a lengths mismatch somewhere.