WPF and MVC4 Web API Internal Server Error 500 on POST - wpf

Question

So I'm attempting to attach to a web api method via a WPF service, but get only a 500 error on anything other than a GET.
WPF call:
using (var client = new HttpClient())
{
var user = new MyUser
{
EntityID = Guid.NewGuid(),
FirstName = "WPF",
LastName = "test"
};
var formatter = new JsonMediaTypeFormatter();
HttpContent content = new ObjectContent<MyUser>(user, formatter);
client.BaseAddress = new Uri("http://localhost:19527/api/");
var response = await client.PostAsJsonAsync("MyUser", content);
//.ContinueWith((postTask) => result = (postTask.Result.Content == null) ? "Could not create user" : "User created successully!");
var r = response.StatusCode;
}'
...and the receiving controller:
public HttpResponseMessage Get(string badgeId)
{
return Request.CreateResponse<bool>(HttpStatusCode.OK, (service.UserByBadge(badgeId) != null));
}
public HttpResponseMessage Put(MyUser user)
{
return Request.CreateResponse<bool>(HttpStatusCode.OK, service.UpsertUser(user));
}
public HttpResponseMessage Post(MyUser user)
{
if (service.UpsertUser(user)) return Request.CreateResponse<MyUser>(HttpStatusCode.OK, service.Get<MyUser>(u => u.BadgeID == user.BadgeID));
return Request.CreateResponse<MyUser>(HttpStatusCode.NoContent, null);
}'
The service on the WebApi controller is a GenericRepository, which is working fine, since the Get method returns as expected. It's only when I use Post that I get the error. Debugging the methods throws the break point in the Get, but not in the Post, so I don't think it's ever being called.
Here's the route config:
routes.MapRoute(
name: "Default",
url: "api/{controller}/{action}/{id}",
defaults: new { controller = "{controller}", action = "{action}", id = UrlParameter.Optional }
);
I've tried different examples from other SO posts, but none appear to address this issue specifically. I'm guessing there's something wrong with how I've constructed the Post() method?
================================================================
RESOLUTION: Model being passed was failing property validations. Why this was causing a 500, not certain. But once I solved for this, API method began working.
If anybody has a "why" explanation, would love to know for future reference.