Controller method to get our own descriptor

I'm wondering why this pythin snippet

def main():
  ControlPort = 9051
  ProcessName = 'tor'

  with Controller.from_port(port = ControlPort) as controller:
    controller.authenticate()
    #print " Tor version %s" % controller.get_version()
    try:
      desc = controller.get_microdescriptor()
      print " flags : %s" % desc.flags()
      print " measured bandwidth : %i" % desc.measured()
    except Exception as exc:
      print exc
      #pass

gives

Tor was unable to provide the descriptor for 'F1BE15429B3CE696D6807F4D4A58B1BFEC45C822'

Does this means that stems asks tor and tor doesn't know itself ?