Web service returns 500 error if I add system.runtime.cache with named cache on server

Question

My webservice doesn't render when I add the following to the web.config:

<system.runtime.caching>
    <memoryCache>
      <namedCaches>
        <add name="testNamedCache"
             cacheMemoryLimitMegabytes="0" 
             physicalMemoryLimitPercentage="0"
             pollingInterval="00:01:00" />
      </namedCaches>
    </memoryCache>
  </system.runtime.caching>

If I remove this, I can view my web service at:

http://localhost:53469/testservice_v1.asmx

It works locally, but when I publish to the live server it fails.

Both are running asp.net 4.0, and app pools are 4.0, both are IIS7.