Trouble calling a ASP.NET MVC 4 WebApi from a Windows Store Application

I've got the following code running in a Windows Store application that is supposed to call one of my WebApis:

using (var client = new HttpClient())
{
    var parms = new Dictionary<string, string>
        {
            {"vinNumber", vinNumber}, 
            {"pictureType", pictureType}, 
            {"blobUri", blobUri}
        };

    HttpResponseMessage response;

    using (HttpContent content = new FormUrlEncodedContent(parms))
    {
        const string url = "http://URL/api/vinbloburis";

        response = await client.PostAsync(url, content);
    }

    return response.StatusCode.ToString();
}

The WebApi code looks like this:

[HttpPost]
public HttpResponseMessage Post(string vinNumber, string pictureType, string blobUri)
{
    var vinEntity = new VinEntity
        {
            PartitionKey = vinNumber,
            RowKey = pictureType, 
            BlobUri = blobUri
        };

    _vinRepository.InsertOrUpdate(vinEntity);

    return new HttpResponseMessage { Content = new StringContent("Success"), StatusCode = HttpStatusCode.OK };
}

Using Fiddler, I've observed the following ... here's what the request looks like:

POST http://URL/api/vinbloburis HTTP/1.1
Content-Type: application/x-www-form-urlencoded
Host: URL
Content-Length: 113
Expect: 100-continue

vinNumber=vinNumber&pictureType=top&blobUri=https%3A%2F%2Fmystorage.blob.core.windows.net%2Fimages%2Fimage.png

But the response is an error with little/no information:

{"Message":"An error has occurred."}

I've even tried locally with the debugger attached to the WebApis and my Post method never catches.

Does anyone see something I've missed here? I feel like I'm looking right at it but not seeing it. I should add that I am able to call an HttpGet method while passing a parameter through the querystring. Right now the problem is only with this HttpPost.

Thanks!

UPDATE: Based on some good comments from folks I'm adding some more details.

I have the default routes configured for WebApis ...

public static void Register(HttpConfiguration config)
{
    config.Routes.MapHttpRoute(
        name: "DefaultApi",
        routeTemplate: "api/{controller}/{id}",
        defaults: new { id = RouteParameter.Optional }
    );
}

Consequently, I think /URL/api/vinbloburis should work. Additionally, as alluded above, I currently have this working with an HttpGet. Here's what's working in the Windows Store app to call the HttpGet WebApi ...

using (var client = new HttpClient())
{
    using (var response = await client.GetAsync(uri))
    {
        if (response.IsSuccessStatusCode)
        {
            var sasUrl = await response.Content.ReadAsStringAsync();
            sasUrl = sasUrl.Trim('"');

            return sasUrl;
        }
    }
}

... and it's calling the following WebApi ...

[HttpGet]
public HttpResponseMessage Get(string blobName)
{
    const string containerName = "images";
    const string containerPolicyName = "policyname";

    _helper.SetContainerPolicy(containerName, containerPolicyName);

    string sas = _helper.GenerateSharedAccessSignature(blobName, containerName, containerPolicyName);

    return new HttpResponseMessage
        {
            Content = new StringContent(sas),
            StatusCode = HttpStatusCode.OK
        };
}

I hope the additional information helps!