RestSharp multipart request in from Unity fails with NRE after successfully sending request

NullReferenceException: Object reference not set to an instance of an object
System.Net.Http.MultipartContent.Dispose (System.Boolean disposing) (at <92a43029e91441ee9df5714eb5dcd209>:0)
System.Net.Http.HttpContent.Dispose () (at <92a43029e91441ee9df5714eb5dcd209>:0)
RestSharp.RequestContent.Dispose () (at <dfd03b4ab8614e549933c37db1618c06>:0)
RestSharp.RestClient.ExecuteRequestAsync (RestSharp.RestRequest request, System.Threading.CancellationToken cancellationToken) (at <dfd03b4ab8614e549933c37db1618c06>:0)
RestSharp.RestClient.ExecuteAsync (RestSharp.RestRequest request, System.Threading.CancellationToken cancellationToken) (at <dfd03b4ab8614e549933c37db1618c06>:0)
TestComponent.TestWebhookSend () (at Assets/Runtime/Scripts/TestComponent.cs:106)

request.Method = Method.Post;
request.AddFile("file1", file.FullName);
request.AddParameter("payload_json", message.ToJsonString(), ParameterType.RequestBody);
var response = await client.ExecuteAsync(request);