AngularJS empty first element in dropdown list

http://stackoverflow.com/questions/14706986/angularjs-empty-first-element-in-dropdown-list

HTML

<div ng-controller="Ctrl">
    <select id="sel" class="input-block-level" ng-model="list_category" ng-options="obj.id as obj.name for obj in list_categories.data">    
        <option value="">Something</option>
    </select>
    
    <p>$scope.list_category: {{list_category}}<p>
<div>

CSS

p {
    margin-top: 10px;
}

JavaScript

var app = angular.module('app', []);

function Ctrl($scope) {  
    $scope.list_categories = {
        data: [{
            id: 'id1',
            name: 'name1'
        }, {
            id: 'id2',
            name: 'name2'
        }]
    };       
}