控制器中的ng-model值未定义为自定义引导下拉提交

Question

这是我的HTML代码

<div ng-controller="formController">
<form>
<div class=btn-group>
    <div class="dropdown top-buttons">
        <button class="btn btn-default dropdown-toggle" type="button" data-toggle="dropdown">
            <span class="menu-item" ng-model="model.shiftID">Shift</span>
            <span class="caret"></span>
        </button>
        <ul class="dropdown-menu" role="menu" aria-labelledby="dropdownMenu1">
            <li><a role="menuitem" href="#">E</a></li>
            <li><a role="menuitem" href="#">L</a></li>
            <li><a role="menuitem" href="#">N</a></li>
        </ul>
    </div>
</div>
<button type="submit" class="btn btn-success select-submit" ng-click="getCall()">GO!</button>
</form> 
</div>

角度JS控制器代码

.controller("formController", function ($scope)
 {
    // when initialized only the initial value is displayed
    // in the alert. But the passed value through 
    // ng-model directive is still undefined
    $scope.model = {shiftID: "init"};
    $scope.getCall = function () {

         alert($scope.model.shiftID)
    }
    .....
 }

我已经完成了其他一些解决方案，但尚未能解决。