How to get rid off the error 500 while using the IpSecurity in the Web.config

Question

This is what I have in the folder that I want to protect from unauthorized users.
I only want that the user IP while logged on to my ASP.NET app has access to the files located in the folder {articles/}

<?xml version="1.0"?>
<configuration> 
  <system.web></system.web>
  <system.webServer>
  <security>
  <ipSecurity allowUnlisted="false">
  <clear/>
  <add ipAddress="98.54.165.26" allowed="true"/>
 
  </ipSecurity>
  </security> 
  </system.webServer>
  </configuration>

But instead of getting an error 403 I get the error 500 the hosting environment is win 2008 IIS7.

Does any one have an idea how to fix this issue?

Thanks