title: Pandas - filling NaNs in Categorical data
body: <p>I am trying to fill missing values (NAN) using the below code</p>
  
  <pre><code>NAN_SUBSTITUTION_VALUE = 1
  g = g.fillna(NAN_SUBSTITUTION_VALUE)
  </code></pre>
  
  <p>but I am getting the following error </p>
  
  <pre><code>ValueError: fill value must be in categories.
  </code></pre>
  
  <p>Would anybody please throw some light on this error.</p>