AngularJS empty value in select

Why AngularJS adds a question mark inside select element? Answer on my blog http://vincentaudebert.github.io/

by Vincent Audebert

HTML

<div ng-controller="selectController">
    <div>
        Select not initialized:
        <select class="form-control" ng-model="selectedBook_noinit" ng-options="book.id as book.title for book in books"></select>
    </div>
    <div>
        Select initialized to a specific value:
        <select class="form-control" ng-model="selectedBook" ng-options="book.id as book.title for book in books" ng-init="selectedBook=0"></select>
    </div>
</div>

JavaScript

angular.module("SelectModule", [])
    .controller("selectController", function selectController($scope) {
    $scope.books = [
        {
            "id": 0,
            "title": "Title Book #1"
        },
        {
            "id": 1,
            "title": "Title Book #2"
        },
        {
            "id": 2,
            "title": "Title Book #3"
        },
    ];
});