diff --git a/system/libraries/Email.php b/system/libraries/Email.php
index 6a8bda7..967a001 100644
--- a/system/libraries/Email.php
+++ b/system/libraries/Email.php
@@ -1035,7 +1035,7 @@
 		if (function_exists('idn_to_ascii') && strpos($email, '@'))
 		{
 			list($account, $domain) = explode('@', $email, 2);
-			$domain = is_php('5.4')
+			$domain = defined('INTL_IDNA_VARIANT_UTS46')
 				? idn_to_ascii($domain, 0, INTL_IDNA_VARIANT_UTS46)
 				: idn_to_ascii($domain);
 			$email = $account.'@'.$domain;
diff --git a/system/libraries/Form_validation.php b/system/libraries/Form_validation.php
index 3444c65..6a97ee5 100644
--- a/system/libraries/Form_validation.php
+++ b/system/libraries/Form_validation.php
@@ -1231,7 +1231,7 @@
 	{
 		if (function_exists('idn_to_ascii') && preg_match('#\A([^@]+)@(.+)\z#', $str, $matches))
 		{
-			$domain = is_php('5.4')
+			$domain = defined('INTL_IDNA_VARIANT_UTS46')
 				? idn_to_ascii($matches[2], 0, INTL_IDNA_VARIANT_UTS46)
 				: idn_to_ascii($matches[2]);
 			$str = $matches[1].'@'.$domain;
diff --git a/user_guide_src/source/changelog.rst b/user_guide_src/source/changelog.rst
index 686f9b8..f743083 100644
--- a/user_guide_src/source/changelog.rst
+++ b/user_guide_src/source/changelog.rst
@@ -7,6 +7,12 @@
 
 Release Date: Not Released
 
+- General Changes
+
+Bug fixes for 3.1.8
+-------------------
+
+-  Fixed a bug where :doc:`Form Validation Library <libraries/form_validation>`, :doc:`Email Library <libraries/email>` tried to use ``INTL_IDNA_VARIANT_UTS46`` when it was undeclared.
 
 Version 3.1.7
 =============