"Authentication credentials were not provided." Error

[ghost]

I've implemented a webhook to get status updates. Testing the webhook via the portal works fine. When trying to create a tracker via the SDK I get an "Authentication credentials were not provided." error. I'm using the example code from the website and the test data for carrier, etc. Here's my code:

try
{
ShippoSDK sdk = new ShippoSDK(
apiKeyHeader: "shippo_test_6fd5xxxxxxxxxxxxxxxxxxxxxxx15ce1e4xxxxxxxxx",
shippoApiVersion: "2018-02-08"
);

Track res1 = await sdk.TrackingStatus.CreateAsync(
tracksRequest: new TracksRequest()
{
    Carrier = "shippo",
    Metadata = "PO 123456",
    TrackingNumber = "SHIPPO_TRANSIT",
},
shippoApiVersion: "2018-02-08");

.
.
.

The error occurs almost immediately on the CreateAsync() call. Any ideas why I need authentication credentials when the SDK objecthas already been created with the authorization token? If I make a call to GetAsync with the same test data, it works fine.

Thanks