diff --git a/forms/TreeDropdownField.php b/forms/TreeDropdownField.php
index ff78b27fe..bdfd3ca12 100755
--- a/forms/TreeDropdownField.php
+++ b/forms/TreeDropdownField.php
@@ -48,8 +48,11 @@ class TreeDropdownField extends FormField {
$id = $this->id();
+ $classes = "TreeDropdownField single";
+ if($this->extraClass()) $classes .= ' ' . $this->extraClass();
+
return <<$title
+
HTML;
}