diff --git a/system/language/english/form_validation_lang.php b/system/language/english/form_validation_lang.php
index 305de37..842c1ae 100644
--- a/system/language/english/form_validation_lang.php
+++ b/system/language/english/form_validation_lang.php
@@ -43,6 +43,7 @@
 $lang['form_validation_valid_emails']		= 'The {field} field must contain all valid email addresses.';
 $lang['form_validation_valid_url']		= 'The {field} field must contain a valid URL.';
 $lang['form_validation_valid_ip']		= 'The {field} field must contain a valid IP.';
+$lang['form_validation_valid_base64']		= 'The {field} field must contain a valid Base64 string.';
 $lang['form_validation_min_length']		= 'The {field} field must be at least {param} characters in length.';
 $lang['form_validation_max_length']		= 'The {field} field cannot exceed {param} characters in length.';
 $lang['form_validation_exact_length']		= 'The {field} field must be exactly {param} characters in length.';
diff --git a/user_guide_src/source/changelog.rst b/user_guide_src/source/changelog.rst
index aa0eae7..5e72982 100644
--- a/user_guide_src/source/changelog.rst
+++ b/user_guide_src/source/changelog.rst
@@ -18,6 +18,7 @@
 -  Fixed a bug (#5545) - :doc:`Session Library <libraries/sessions>` crashed due to a caching-related error with the 'files' driver.
 -  Fixed a bug (#5571) - :doc:`XML-RPC Library <libraries/xmlrpc>` had a typo that triggered an ``E_WARNING`` message on PHP 7.2.
 -  Fixed a bug (#5587) - :doc:`Database Forge <database/forge>` method ``create_table()`` generated an ``E_WARNING`` message.
+-  Fixed a bug (#5590) - :doc:`Form Validation Library <libraries/form_validation>` rule **valid_base64** didn't have a default error message.
 
 Version 3.1.9
 =============