Material-ui: RadioButtonGroup fails

As soon as i use RadioButtonGroup i get 'Cannot read property ‘hasOwnProperty’ of undefined’
pointing to

_hasCheckAttribute: function _hasCheckAttribute(radioButton) {
return radioButton.props.hasOwnProperty('checked') && radioButton.props.checked;
},

Using div instead the radio buttons show, but not work of course. Using the static example from the webpage works too. The problem seems to be the map thing.

function renderRadios(answers) {

return   <RadioButtonGroup name="shipSpeed" defaultSelected="Berlin">

{answers.map((answer) => {
      return    <RadioButton
        value={answer.score}
        label={answer.answer}
        style={{marginBottom:16}}
        />
    })
}     </RadioButtonGroup>
}

I´ve posted this on github as well, but didn´t get any response.
thanks
Benjamin