PerformanceManager::queryPerf is returning null for resource pool performance metrics

I'm using PerformanceManager::queryPerf(PerfQuerySpec[]) to query perf.cpu.usagemhz.average and perf.mem.consumed.average for ResourcePool. The same program can get return value for lower version VCs, but get null for VC 6.0. In vSphere, using same account that is used for API call, I can get performance data for the ResourcePool I'm trying to query.

Did anyone else used that API for VC 6.0 target? Is it working fine?

Thanks!