javascript - Angular遍历Radio选项，如何设置某个选项为默认选中状态?

<script type="text/javascript">
    var testObj = angular.module('testApp',[]);
    testObj.controller("testCtr",function($scope){
        $scope.orgTypes = ['其他','医院','养老院'];
        $scope.viewModel.orgType = $scope.orgTypes[0];
    });
</script>

写了之后，Angular遍历出错了

TypeError: Cannot set property 'orgType' of undefined
    at new <anonymous> (test4NgRadio.html:24)
    at Object.invoke (angular.js:4862)
    at $controllerInit (angular.js:10717)
    at nodeLinkFn (angular.js:9594)
    at compositeLinkFn (angular.js:8903)
    at compositeLinkFn (angular.js:8906)
    at publicLinkFn (angular.js:8768)
    at angular.js:1847
    at Scope.$eval (angular.js:18017)
    at Scope.$apply (angular.js:18117)

$scope.viewModel={
    orgType = $scope.orgTypes[0]
}