diff --git a/includes/form.inc b/includes/form.inc
index fa52b74..d97b6c7 100644
--- a/includes/form.inc
+++ b/includes/form.inc
@@ -260,6 +260,22 @@ function bootstrap_button($variables) {
   $label = $element['#value'];
   element_set_attributes($element, array('id', 'name', 'value', 'type'));
 
+  // If a button type class isn't present then add in default.
+  $button_classes = array(
+    'btn-default',
+    'btn-primary',
+    'btn-success',
+    'btn-info',
+    'btn-warning',
+    'btn-danger',
+    'btn-link',
+  );
+  $class_intersection = array_intersect($button_classes, $element['#attributes']['class']);
+  if (empty($class_intersection)) {
+    $element['#attributes']['class'][] = 'btn-default';
+  }
+
+  // Add in the button type class.
   $element['#attributes']['class'][] = 'form-' . $element['#button_type'];
   if (!empty($element['#attributes']['disabled'])) {
     $element['#attributes']['class'][] = 'form-button-disabled';
diff --git a/includes/theme.inc b/includes/theme.inc
index 9a9e45a..bc8dc7c 100644
--- a/includes/theme.inc
+++ b/includes/theme.inc
@@ -134,6 +134,9 @@ function theme_bootstrap_btn_dropdown($variables) {
   if (isset($variables['type'])) {
     $type_class = ' btn-'. $variables['type'];
   }
+  else {
+    $type_class = ' btn-default';
+  }
 
   // Start markup
   $output = '<div'. drupal_attributes($variables['attributes']) .'>';