在使用react-boostrap的组件的时候结果报了一个错误...

 Invariant Violation: addComponentAsRefTo(...): Only a ReactOwner can have refs. This usually means that you're trying to add a ref to a component that doesn't have an owner (that is, was not created inside of another component's `render` method). Try rendering this component inside of a new top-level component which will hold the ref.

代码大概是这个样子的：

 var React = require('React');
var ReactBootstrap = require('react-bootstrap');
var DropdownButton = ReactBootstrap.DropdownButton;
var MenuItem = ReactBootstrap.MenuItem;
var ButtonToolbar = ReactBootstrap.ButtonToolbar;

var Header = React.createClass({
    render: function(){
        return (
            <ButtonToolbar>
                <DropdownButton bsStyle="link" title="default">
                    <MenuItem eventKey="1">active</MenuItem>
                    <MenuItem divider />
                </DropdownButton>
            </ButtonToolbar>
        );
    }
});

module.exports = Header;

请问是为什么呢...