Exception when adding log4net config

The issue was that I had the <startup> xml node in the app.config file at the start rather than the end of the file. It needs to be the last thing in the app.config file.

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  // lots of other stuff here...
  <startup> 
    <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.5.1"/>
  </startup>
</configuration>