Golang Google Maps API到达

I've written some code in Golang to get directions from one place to another using the Google Maps API. According to the Godoc, both the DepartureTime and ArrivalTime parameters are optional. However, if I try to make any request without specifying the departure time, the code fails, giving me the following error message: INVALID_REQUEST - Invalid request. Missing the 'departure_time' parameter. Here's the relevant code:

client, err := maps.NewClient(maps.WithAPIKey(apiKey), maps.WithRateLimit(2))
check(err, "new maps client")

r := &maps.DirectionsRequest{
    Origin:       origin,
    Destination:  destination,
    ArrivalTime:  arrivalTime,
    Alternatives: alternatives,
    Mode:         maps.TravelModeDriving,
}

lookupTrafficModel(trafficModel, r)

if avoid != "" {
    lookupAvoidPoints(avoid, r)
}

//THIS LINE IS WHERE THE ERROR IS THROWN
routes, waypoints, err := client.Directions(context.Background(), r)
check(err, "getting directions")

fmt.Println(waypoints)
fmt.Println(routes)

Note that if I comment out the ArrivalTime parameter and instead use DepartureTime: "now", (as per the Godoc), this code works as expected.

It would appear this is a limitation of the Google Maps API, not the Go client. For those who see this question in the future, experiment with different combinations of parameters (leaving different ones out) until it works.