TypeError: Cannot read property 'toUpperCase' of undefined - NavigationClose

[raptoria]

I'm getting the following error
Warning: React.createElement: type should not be null or undefined. It should be a string (for DOM elements) or a ReactClass (for composite components).
warning.js:48 Warning: Only functions or strings can be mounted as React components.
createRouter.js:275 Uncaught TypeError: Cannot read property 'toUpperCase' of undefined

If I remove NavigationClose from the AppBar, the error goes away.

currently running React v 0.13.1
react-router v 0.13.2
material-ui v 0.10.4

Code:

    import React from 'react';
    import styles from './Header.css';
    import mui from 'material-ui';

    let IconButton = mui.IconButton;
    let NavigationClose = mui.NavigationClose;
    let FlatButton = mui.FlatButton;
    let ThemeManager = new mui.Styles.ThemeManager();
    let AppBar = mui.AppBar;

    let injectTapEventPlugin = require('react-tap-event-plugin');
    injectTapEventPlugin();


    class Header extends React.Component{

      getChildContext() {
        return {
          muiTheme: ThemeManager.getCurrentTheme()
        };
      }

      render() {
        return (
          <AppBar
          title="Title"
          iconElementLeft={<IconButton><NavigationClose /></IconButton>}
          iconElementRight={<FlatButton label="Save" />} />
        );
      }

    }

    Header.childContextTypes = {
      muiTheme: React.PropTypes.object
    };

    export default Header;

[raptoria]

Figured it out:
let CloseButton = require('material-ui/lib/svg-icons/navigation/close');