diff --git a/src/InvalidContentTypeException.php b/src/InvalidContentTypeException.php
index 2c82d4d..4458785 100644
--- a/src/InvalidContentTypeException.php
+++ b/src/InvalidContentTypeException.php
@@ -20,5 +20,5 @@
  */
 class InvalidContentTypeException extends \Exception
 {
-    private $message = 'Content type should be a string or array of NodeElement objects';
+    protected $message = 'Content type should be a string or array of NodeElement objects';
 }