diff --git a/theme/system/button.func.php b/theme/system/button.func.php
index 02531a1..615e8c3 100644
--- a/theme/system/button.func.php
+++ b/theme/system/button.func.php
@@ -10,7 +10,11 @@
 function bootstrap_button($variables) {
   $element = $variables['element'];
   $label = $element['#value'];
-  element_set_attributes($element, array('id', 'name', 'value', 'type'));
+
+  // Drupal buttons should be of type 'submit'.
+  // @see https://www.drupal.org/node/2540452
+  $element['#attributes']['type'] = 'submit';
+  element_set_attributes($element, array('id', 'name', 'value'));

   // If a button type class isn't present then add in default.
   $button_classes = array(