NaN is not recognized in pandas after np.where clause. Why? Or is it a bug?
<p>NaN is not recognized in pandas after np.where clause. Why? Or is it a bug?</p>

<p>The last line of this code should be "True"</p>

<pre><code>In [1]: import pandas as pd
In [2]: import numpy as np   
In [3]: a=pd.Series([1,np.nan])    
In [4]: b=pd.DataFrame(["a","b"])  
In [5]: b["1"]=np.where(
            a.isnull(),
            np.nan,
            "Hello"
        )   
In [6]: b
Out[6]:
   0      1
0  a  Hello
1  b    nan    
In [7]: b[1].isnull()
Out[7]:
0    False
1    False
Name: 1, dtype: bool
</code></pre>