RegisterVM_Task doesn't work properly without the "pool" param

from the vSphere API doc, only path and asTemplate are required, and vCLI vmware-cmd works properly without specifying resource pool. However, in here, without the pool argument, this is what I get.

(vim.TaskInfo) {
   dynamicType = <unset>,
   dynamicProperty = (vmodl.DynamicProperty) [],
   key = 'haTask-ha-folder-vm-vim.Folder.registerVm-307940434',
   task = 'vim.Task:haTask-ha-folder-vm-vim.Folder.registerVm-307940434',
   description = <unset>,
   name = vim.Folder.RegisterVm,
   descriptionId = 'Folder.registerVm',
   entity = 'vim.Folder:ha-folder-vm',
   entityName = 'vm',
   locked = (vim.ManagedEntity) [],
   state = 'error',
   cancelled = false,
   cancelable = false,
   error = (vmodl.fault.InvalidArgument) {
      dynamicType = <unset>,
      dynamicProperty = (vmodl.DynamicProperty) [],
      msg = 'A specified parameter was not correct. \npool',
      faultCause = <unset>,
      faultMessage = (vmodl.LocalizableMessage) [],
      invalidProperty = u'pool'
   },
   result = <unset>,
   progress = <unset>,
   reason = (vim.TaskReasonUser) {
      dynamicType = <unset>,
      dynamicProperty = (vmodl.DynamicProperty) [],
      userName = 'root'
   },
   queueTime = 2015-02-04T12:40:42.342862Z,
   startTime = 2015-02-04T12:40:42.343253Z,
   completeTime = 2015-02-04T12:40:42.343679Z,
   eventChainId = 307940434,
   changeTag = <unset>,
   parentTaskKey = <unset>,
   rootTaskKey = <unset>
}