Covariance matrix failed if not all option get a result

I created a very simple form with some radio buttons.
I submitted it and tried to fill out one. Then I went to results, I would like to see the analysis and the covariance matrix. And there I get a little bug : warning: Invalid argument supplied for foreach() in c:\archivos de programa\easyphp1-7\www\drupal460\modules\webform\webform.inc on line 913.
In fact, this only happen when all option of a form, didn't have at least one result.