title: Unable to handle NaN in pandas dataframe
body: <p>I have a pandas dataframe with a variable, which, when I print it, shows up as mostly containing NaN. It is of dtype object. However, when I run the isnull function, it returns "FALSE" everywhere. I am wondering why the NaN values are not encoded as missing, and if there is any way of converting them to missing values that are treated properly. </p>
  
  <p>Thank you. </p>