KSP Version: 1.11

I am trying to set SAS mode to retrograde, but getting the following error:

Python code:
import krpc
import time
conn = krpc.connect(name='SpaceX Booster Landing')
vessel = conn.space_center.active_vessel
vessel.control.sas = True
time.sleep(2)
vessel.control.sas_mode = conn.space_center.SASMode.retrograde