angularjs select ng-options，加载时无法选择正确的选项

Question

JS：

    $scope.tasks = [{ Name: "Deliverable", Id: "Deliverable" },
                    { Name: "Email", Id: "Email" }]
    $scope.currentTask = {Subject: "Deliverable"}

HTML：

<select ng-model="currentTask.Subject" ng-options="task.Id as task.Name for task in tasks"></select>

结果： 页面加载时未选择任何选项。但下拉效果很好。我错过了什么吗？