title: Pandas fillna throws ValueError: fill value must be in categories
body: <p>Discription: both features are in categorical dtypes. and i used this code in a different kernal of  same 
  dateset was working fine, the only difference is the features are in flote64. later i have converted these feature dtypes into Categorical
  because all the features in the dataset represents categories.</p>
  
  <p>Below is the code:</p>
  
  <pre><code>AM_train['product_category_2'].fillna('Unknown', inplace =True)
  AM_train['city_development_index'].fillna('Missing', inplace =True)
  </code></pre>