angularjs object is not a function error

I am trying to send a repeater array items to web api controller method as parameter. When I click the button containing the function its giving me this error in console and nothing happens.

object is not a function

I guess I'm passing the orders array in a wrong way. How to do that correctly.

My ng-repeat

    <table class="table table-bordered table-hover" style="width:800px" data-ng-model="orderProduct">
            <tr data-ng-repeat="order in orders">
                <td>{{selectedProduct.pname}}</td>
                <td>{{order.pid}}</td>
                <td>{{order.oid}}</td>
                <td>{{order.qty}}</td>
                <td>{{order.total}}</td>
            </tr>
        </table>
        <input type="button" data-ng-click="addOrder()" class="btn btn-danger" data-ng-disabled="!orders.length" value="Submit Orders" />

My controller method $scope

    $scope.addOrder = function () {
            var orders = this.orders;
            $http.post('/api/OrderDetails/', orders).success(function (data) {
                alert("Added Successfully!!");
                $scope.addMode = false;

            }).error(function (data) {
                $scope.error = "An Error has occured while Adding Order details! " + data;

            });
        };