Cache.EffectivePrivateBytesLimit set to '-1' even though value specified in web.config

Question

I have entered the value in web.config for the cache size as below:

<system.web>
   <caching>

      <cache privateBytesLimit="10000000" privateBytesPollTime="00:01:00" />

   </caching>

However, when I check the value of System.Web.HttpRuntime.Cache.EffectivePrivateBytesLimit, it is set to -1. Any ideas why?