CPU & Memory usage

Hi

I'm using this code to get the overall usage of CPU & Memory, but it returns zero. When i try to print name (i.e. myVcHostSystem.name) it prints properly. but when i try to get quickstatistics, it is returning zero. Can anyone help?

for each ( myVcHostSystem in cluster.host ) 

    { 

      myClusterOverallCpuUsage += myVcHostSystem.summary.quickStats.overallCpuUsage;

      myClusterOverallMemoryUsage += myVcHostSystem.summary.quickStats.overallMemoryUsage; 

    }