warning: Division by zero (Line 361)

I'm getting a "warning: Division by zero ... on line 361" after clicking on Optimize.

This is the code, but i can't figure it out:

    // prominence = ($totalwords - (($positionsum - 1) / $positionsnum)) * (100 / $totalwords)
    $ret['keyword_prominence'] = 0;
    if($ret['keyword_count']) {
      $ret['keyword_prominence'] = ($ret['word_count'] - (($ret['keyword_positionsum'] - 1) / $ret['keyword_count'])) * (100 / $ret['word_count']);
    }