415 error when performing HttpClient.GetAsync

Question

Ok, I've been beating my head against this one for hours, read the docs, and googled the hell out of it. I'm stumped. I'm fairly new to ASP.Net Core/C# and am trying to make a fairly straightforward HTTP Get call to a remote service from an object in my web app. The service requires that I set an "APIKey" header to authenticate against their servers, but is otherwise fairly vanilla and is returning a generic JSON collection. Rather than receiving the JSON, I'm getting a failed status with the reason phrase "Unsupported Media Type" and the status code "415". I can query the service with no problems in Postman.

The RequestMessage in the response is :

{Method: GET, RequestUri: 'url_removed_for_stack', Version: 2.0, Content: <null>, Headers:
{
  Accept: application/json
  Accept-Encoding: gzip
  Accept-Encoding: deflate
  Accept-Encoding: gzip
  Accept-Encoding: deflate
  APIKey: apikey_removed_for_stack
}}

The calling method, in its entirety:

 private async Task<JsonValue> GetAsync(string uri, string apiKey, int timeout)
        {
            var handler = new HttpClientHandler()
            {
                AutomaticDecompression = DecompressionMethods.GZip | DecompressionMethods.Deflate
            };

            using (var client = new HttpClient(handler))
            {
                //remove the default header
                client.DefaultRequestHeaders.Clear();
                client.Timeout = TimeSpan.FromMilliseconds(timeout);

                client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
                client.DefaultRequestHeaders.AcceptEncoding.Add(new StringWithQualityHeaderValue("gzip"));
                client.DefaultRequestHeaders.AcceptEncoding.Add(new StringWithQualityHeaderValue("deflate"));
                client.DefaultRequestHeaders.Add("APIKey", apiKey);

                try
                {
                    var response = await client.GetAsync(uri);
                    if (!response.IsSuccessStatusCode)
                    {
                        isError = true;
                        errorMessage = "GetAsync Response Error: " + response.ReasonPhrase;
                        return await Task.FromResult(0);
                    }

                    var content = await response.Content.ReadAsStringAsync();
                    return await Task.Run(() => JsonValue.Parse(content));
                }
                catch (Exception ex)
                {
                    isError = true;
                    errorMessage = ex.ToString();
                    return await Task.FromResult(0);
                }
            }           
        }

What am I missing?