diff --git a/includes/common.inc b/includes/common.inc
index 9b18292f66c83bdd65d6bfde885d6140aa2a9ef2..f7a6ce3a4199fc43af8347c2595b53e94d33056c 100644
--- a/includes/common.inc
+++ b/includes/common.inc
@@ -2235,7 +2235,9 @@ function drupal_attributes(array $attributes = array()) {
  * @param array $options
  *   An associative array of additional options, with the following elements:
  *   - 'attributes': An associative array of HTML attributes to apply to the
- *     anchor tag.
+ *     anchor tag. If element 'class' is included, it must be an array; 'title'
+ *     must be a string; other elements are more flexible, as they just need
+ *     to work in a call to drupal_attributes($options['attributes']).
  *   - 'html' (default FALSE): Whether $text is HTML or just plain-text. For
  *     example, to make an image tag into a link, this must be set to TRUE, or
  *     you will see the escaped HTML image tag.