diff -u b/core/includes/form.inc b/core/includes/form.inc
--- b/core/includes/form.inc
+++ b/core/includes/form.inc
@@ -3864,8 +3864,20 @@
   if (!empty($element['#autocomplete_path']) && drupal_valid_path($element['#autocomplete_path'])) {
     $element['#attributes']['class'][] = 'form-autocomplete';
     $element['#attached']['library'][] = array('system', 'drupal.autocomplete');
-    // Provide a data attribute for the JavaScript behavior to bind to.
-    $element['#attributes']['data-autocomplete-path'] = url($element['#autocomplete_path'], array('absolute' => TRUE));
+    // Provide a hidden element for the JavaScript behavior to bind to. Since
+    // this element is for client-side functionality only, and we don't want to
+    // collect any input from it, use #theme='hidden' instead of #type='hidden'.
+    // @todo Refactor autocomplete.js to accept Drupal.settings instead of
+    //   requiring extraneous markup.
+    $element['autocomplete'] = array(
+      '#theme' => 'hidden',
+      '#attributes' => array(
+        'id' => $element['#id'] . '-autocomplete',
+        'value' => url($element['#autocomplete_path'], array('absolute' => TRUE)),
+        'class' => array('autocomplete'),
+        'disabled' => 'disabled',
+      ),
+    );
   }
   return $element;
 }
@@ -4084,20 +4096,8 @@
   if (!empty($element['#autocomplete_path']) && drupal_valid_path($element['#autocomplete_path'])) {
     $element['#attributes']['class'][] = 'form-autocomplete';
     $element['#attached']['library'][] = array('system', 'drupal.autocomplete');
-    // Provide a hidden element for the JavaScript behavior to bind to. Since
-    // this element is for client-side functionality only, do not process input.
-    // @todo Refactor autocomplete.js to accept Drupal.settings instead of
-    //   requiring extraneous markup.
-    $element['autocomplete'] = array(
-      '#type' => 'hidden',
-      '#input' => FALSE,
-      '#value' => url($element['#autocomplete_path'], array('absolute' => TRUE)),
-      '#disabled' => TRUE,
-      '#attributes' => array(
-        'class' => array('autocomplete'),
-        'id' => $element['#id'] . '-autocomplete',
-      ),
-    );
+    // Provide a data attribute for the JavaScript behavior to bind to.
+    $element['#attributes']['data-autocomplete-path'] = url($element['#autocomplete_path'], array('absolute' => TRUE));
   }
   return $element;
 }