diff --git a/lib/Calendar.php b/lib/Calendar.php
index ebab761c..782f2f84 100644
--- a/lib/Calendar.php
+++ b/lib/Calendar.php
@@ -12,6 +12,13 @@
  */
 abstract class Kronolith_Calendar
 {
+    /**
+     * The background.
+     *
+     * @var string
+     */
+    protected $_background;
+
     /**
      * Constructor.
      *