React.renderComponentToString failing case when component children is a function

``` javascript
var Hello = React.createClass({
    componentDidMount: function() {},
    render: function() {
        return this.props.children();
    }
});

React.renderComponentToString(
    Hello(null, function() { return React.DOM.div(null, "Hello"); }), 
    function(s) { console.log(s); }
);
```

Results in:

```
Uncaught TypeError: Cannot read property 'firstChild' of undefined 
```
