CustomizeVMTask java api works for vCenter mob but not for ESX mob. Why?

Hi,

I need to change the static ip of VM using code.

CheckCustomizationSpec and CustomizeVMTask java api's work for vCenter mob but not for ESX mob. Why?

For ESX mob(url),  CheckCustomizationSpec throws the error "javax.xml.ws.soap.SOAPFaultException: The operation is not supported on the object."

For vCenter mob(url), the customization works fine when appropriate objects are passed.

Any ideas?