Swagger c# generation fails

Description

When sending the swagger json to https://generator.swagger.io/api/gen/clients/csharp
we get ;
Fri Mar 02 2018 16:48:49.0412 [ERROR] TID: 9 GenerateCsharp ; StatusCode: 400, ReasonPhrase: 'Bad Request', Version: 1.1, Content: System.Net.Http.StreamContent, Headers:
{
Access-Control-Allow-Origin: *
Access-Control-Allow-Methods: GET, POST, DELETE, PUT
Access-Control-Allow-Headers: Content-Type
Connection: close
Date: Fri, 02 Mar 2018 16:48:47 GMT
Server: Jetty(9.2.9.v20150224)
Content-Length: 106
Content-Type: application/json
}

Swagger-codegen version

not sure - current live on website

Swagger declaration file content or url

https://gist.github.com/jemprince/a73026aee1eadb8d09c12fecbe65b63b

Command line used for generation

Steps to reproduce

codeGenPath="https://generator.swagger.io/api/gen/";
...
using (var client2 = this.iClientWithProxy.Create())
{
client2.BaseAddress = new Uri(codeGenPath);
client2.DefaultRequestHeaders.Accept.Clear();
client2.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

            using (var request = new HttpRequestMessage(HttpMethod.Post, genPath))
            {
            request.Content = new StringContent(swaggerSpec, Encoding.UTF8, "application/json");

...
(swaggerSpec as above)

Related issues/PRs

Start by reproducing the 400 response at https://generator.swagger.io/api/gen/clients/csharp with the supplied Swagger JSON and identify which part of the declaration the service rejects. Done means the same declaration is accepted and a C# client is generated; the issue does not name a repository file or test to run.