[bootstrap][policy][7.x-3.x] CSS Classes (Fatal error: [] operator not supported for strings)

changing bootstrap/theme/system/image.vars.php

<?php
/**
 * Implements hook_preprocess_image().
 */
function bootstrap_preprocess_image(&$variables) {
  // Add image shape, if necessary.
  if ($shape = theme_get_setting('bootstrap_image_shape')) {
    $variables['attributes']['class'][] = $shape;
  }
  // Add responsiveness, if necessary.
  if ($shape = theme_get_setting('bootstrap_image_responsive')) {
    $variables['attributes']['class'][] = 'img-responsive';
  }
}
?>

to:

<?php
/**
 * Implements hook_preprocess_image().
 */
function bootstrap_preprocess_image(&$variables) {
  // Add image shape, if necessary.
  if ($shape = theme_get_setting('bootstrap_image_shape')) {
    $variables['attributes']['class'] = $shape;
  }
  // Add responsiveness, if necessary.
  if ($shape = theme_get_setting('bootstrap_image_responsive')) {
    $variables['attributes']['class'] = 'img-responsive';
  }
}
?>

... solved it. But this will overwrite the complete values regarding the key ['class'], not sure if this is wanted. Also I don't know why we have a double if here instead of an elseif ... Let me know what you think. Thanks ...