diff --git a/includes/form.inc b/includes/form.inc
index e749239..914715f 100644
--- a/includes/form.inc
+++ b/includes/form.inc
@@ -3134,7 +3134,7 @@ function form_process_date($element) {
  * Validates the date type to prevent invalid dates (e.g., February 30, 2006).
  */
 function date_validate($element) {
-  if (!checkdate($element['#value']['month'], $element['#value']['day'], $element['#value']['year'])) {
+  if (!checkdate((int) $element['#value']['month'], (int) $element['#value']['day'], (int) $element['#value']['year'])) {
     form_error($element, t('The specified date is invalid.'));
   }
 }