Why does virt-who not work with VMware ESXi hosts ?

Issue

• The virt-who does not work with ESXi host and throws below traceback,

  @virt-who.py:186 - Error in communication with virt backend, trying to recover:
Traceback (most recent call last):
  File "/usr/share/virt-who/virt-who.py", line 180, in _send
    virtualGuests = self.virt.getHostGuestMapping()
  File "/usr/share/virt-who/vsphere.py", line 268, in getHostGuestMapping
    self.scan()
  File "/usr/share/virt-who/vsphere.py", line 216, in scan
    for propSet in obj.propSet:
AttributeError: 'unicode' object has no attribute 'propSet'
2013-03-26 10:47:14,098 [ERROR]  @virt-who.py:189 - Unable to recover, retry in 60 seconds.