Can't get Google travel time arrival time to work

I’ve been trying to get google to tell me when to leave for my next calendar event but whenever I try to use a sensor for the arrival_time option the API returns an error.
Here’s my config:

- platform: google_travel_time
  name: Phone To Next Cal Event
  scan_interval:
    days: 9999
  api_key: !secret google_maps_api_key
  origin: device_tracker.maddies_iphone
  destination: sensor.google_cal_location
  options:
    mode: transit
    arrival_time: sensor.google_cal_time

and the error:

Error doing job: Future exception was never retrieved
Traceback (most recent call last):
  File "/usr/local/lib/python3.7/concurrent/futures/thread.py", line 57, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/usr/src/homeassistant/homeassistant/components/google_travel_time/sensor.py", line 108, in run_setup
    hass, name, api_key, origin, destination, options)
  File "/usr/src/homeassistant/homeassistant/components/google_travel_time/sensor.py", line 144, in __init__
    self.update()
  File "/usr/src/homeassistant/homeassistant/util/__init__.py", line 224, in wrapper
    result = method(*args, **kwargs)
  File "/usr/src/homeassistant/homeassistant/components/google_travel_time/sensor.py", line 225, in update
    self._origin, self._destination, **options_copy)
  File "/usr/local/lib/python3.7/site-packages/googlemaps/client.py", line 356, in wrapper
    result = func(*args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/googlemaps/distance_matrix.py", line 130, in distance_matrix
    return client._request("/maps/api/distancematrix/json", params)
  File "/usr/local/lib/python3.7/site-packages/googlemaps/client.py", line 253, in _request
    result = self._get_body(response)
  File "/usr/local/lib/python3.7/site-packages/googlemaps/client.py", line 267, in _get_body
    raise googlemaps.exceptions.HTTPError(response.status_code)
googlemaps.exceptions.HTTPError: HTTP Error: 400

For testing I’ve hardcoded the value of sensor.google_cal_time to be 09:00:00.

I’ve tried just about every format I can think of to try and get it to work. Any help would be very appreciated. Thanks!