diff --git a/system/libraries/Form_validation.php b/system/libraries/Form_validation.php
index 9d16602..05de596 100644
--- a/system/libraries/Form_validation.php
+++ b/system/libraries/Form_validation.php
@@ -831,7 +831,7 @@
 						// DEPRECATED support for non-prefixed keys
 						&& FALSE === ($line = $this->CI->lang->line($rule, FALSE)))
 					{
-						$line = $this->CI->lang->line('form_validation_error_message_not_set');
+						$line = $this->CI->lang->line('form_validation_error_message_not_set').'('.$rule.')';
 					}
 				}
 				else
diff --git a/user_guide_src/source/changelog.rst b/user_guide_src/source/changelog.rst
index ef3d2af..99e4de5 100644
--- a/user_guide_src/source/changelog.rst
+++ b/user_guide_src/source/changelog.rst
@@ -362,6 +362,7 @@
       -  Added support for custom error messages per field rule.
       -  Added support for callable rules when they are passed as an array.
       -  Added support for non-ASCII domains in **valid_email** rule, depending on the Intl extension.
+      -  Changed the debug message about an error message not being set to include the rule name it is about.
 
    -  :doc:`Caching Library <libraries/caching>` changes include: