title: How to skip NaN values when splitting up a column
body: <p>I am trying to split a column up into two columns based on a delimeter. The column presently has text that is separated by a '-'. Some of the values in the column are NaN, so when I run the code below, I get the following error message: ValueError: Columns must be same length as key.</p>
  <p>I don't want to delete the NaN values, but am not sure how to skip them so that this splitting works.</p>
  <p>The code I have right now is:</p>
  <pre><code>df[['A','B']] = df['A'].str.split('-',expand=True)
  </code></pre>