Error with Asp.NET Web.API + AngularJS

I have my method C# so

[ResponseType(typeof(Categoria))]
public async Task<IHttpActionResult> Post(Categoria model) {
    if (!ModelState.IsValid) {
        return BadRequest(ModelState);
    }
    if (_repositorio.InsertOrUpdate(model, out Res)) {
        _repositorio.Save();
        return CreatedAtRoute("DefaultApi", new { id = model.Id }, model);
    }
    return BadRequest("Erro ao tentar salvar, tente novamente mais tarde");    
}

I have my factory like this:

.factory("categoriasService", function($http, config, $q) {
    var _postItem = function(record) {
        var deferred = $q.defer();
        $http.post(config.baseUrl + "/api/Categoria/Post", record).then(
            function(result) {
                deferred.resolve(result.data);
            },
            function (erroResult) {
                deferred.reject();
            }
        );
        return deferred.promise;
    };
    return {            
        postItem: _postItem
    };
})

When calling in my controller :

.controller("newCategoriaCtrl", [
    "$scope", "$http", "$window", "categoriasService", "modalConfirmationService", "$routeParams",
    function ($scope, $http, $window, categoriasService, modalConfirmationService, $routeParams) {            
        if ($routeParams.id !== undefined) {
            categoriasService.find($routeParams.id).then(function (result) {
                $scope.newCategoria = result;
            });
            $scope.titleAcao = "Alterar";
        } else {
            $scope.newCategoria = { Id: 0 };
            $scope.titleAcao = "Cadastrar";
        }

        $scope.save = function() {
            //console.log($scope.newCategoria);
            categoriasService.postItem($scope.newCategoria)
            .then(function (newCategoria) {
                console.log(newCategoria);//aqui o objeto está sempre vazio por que acontece o erro 500.
                modalConfirmationService.getModalInstance("Sucesso", "Dados salvos com sucesso!");
            },
            function() {
                modalConfirmationService.getModalInstance("Erro", "Não foi possível executar sua ação, tente novamente mais tarde.");
            })
            .then(function() {
                $window.location = "#";
            });
        };
    }
]);

When I call the application, without activating the debug in the browser console, error 500 always occurs and does not save the information in the database.

http://localhost:54100/api/Categoria/Post 500 (Internal Server Error)

But if I activate debug mode in VS, it gives the same error in the console, but the information is saved in the database.

What's wrong? Do I have to configure something in the web.api app?