I am attempting to configure ELMAH to filter 404 errors and I am running into difficulties with the XML-provided filter rules in my Web.config file. I followed the tutorial here and here and added an <is-type binding="BaseException" type="System.IO.FileNotFoundException" /> declaration under my <test><or>... declaration, but that completely failed.

When I tested it locally I stuck a breakpoint in protected void ErrorLog_Filtering() {} of the Global.asax found that the System.Web.HttpException that gets fired by ASP.NET for a 404 doesn't have a base type of System.IO.FileNotFound, but rather it is simply a System.Web.HttpException.

Next I decided to try a <regex binding="BaseException.Message" pattern="The file '/[^']+' does not exist" /> in the hopes that any exception matching the pattern "The file '/foo.ext' does not exist" would get filtered, but that too having no effect. As a last resort I tried <is-type binding="BaseException" type="System.Exception" />, and even that is entirely disregarded.

I'm inclined to think there's a configuration error with ELMAH, but I fail to see any. Am I missing something blatantly obvious?

Here's the relevant stuff from my web.config:

<configuration>
  <configSections>
    <sectionGroup name="elmah">
      <section name="security" requirePermission="false" type="Elmah.SecuritySectionHandler, Elmah"/>
      <section name="errorLog" requirePermission="false" type="Elmah.ErrorLogSectionHandler, Elmah"/>
      <section name="errorMail" requirePermission="false" type="Elmah.ErrorMailSectionHandler, Elmah"/>
      <section name="errorFilter" requirePermission="false" type="Elmah.ErrorFilterSectionHandler, Elmah" />
    </sectionGroup>
  </configSections>
  <elmah>
    <errorFilter>
      <test>
        <or>
          <equal binding="HttpStatusCode" value="404" type="Int32" />
          <regex binding="BaseException.Message" pattern="The file '/[^']+' does not exist" />
        </or>
      </test>
    </errorFilter>
    <errorLog type="Elmah.XmlFileErrorLog, Elmah" logPath="~/App_Data/logs/elmah" />
  </elmah>
  <system.web>
    <httpModules>
      <add name="ErrorFilter" type="Elmah.ErrorFilterModule, Elmah"/>
      <add name="ErrorLog" type="Elmah.ErrorLogModule, Elmah"/>
    </httpModules>
  </system.web>
  <system.webServer>
    <modules>
      <add name="ErrorFilter" type="Elmah.ErrorFilterModule, Elmah"/>
      <add name="ErrorLog" type="Elmah.ErrorLogModule, Elmah" />
    </modules>
  </system.webServer>
</configuration>