diff --git a/application/libraries/Captcha_builder.php b/application/libraries/Captcha_builder.php
index d06f2aeb..ce61b9d0 100644
--- a/application/libraries/Captcha_builder.php
+++ b/application/libraries/Captcha_builder.php
@@ -41,6 +41,10 @@ class Captcha_builder
      * @var array
      */
     protected $lineColor = null;
+    /**
+     * @var array
+     */
+    protected $background = null;
     /**
      * @var array
      */