diff --git a/system/libraries/Form_validation.php b/system/libraries/Form_validation.php
index 61f0298..c39b65d 100644
--- a/system/libraries/Form_validation.php
+++ b/system/libraries/Form_validation.php
@@ -1229,9 +1229,9 @@
 	 */
 	public function valid_email($str)
 	{
-		if (function_exists('idn_to_ascii') && $atpos = strpos($str, '@'))
+		if (function_exists('idn_to_ascii') && sscanf($str, '%[^@]@%s', $name, $domain) === 2)
 		{
-			$str = substr($str, 0, ++$atpos).idn_to_ascii(substr($str, $atpos));
+			$str = $name.'@'.idn_to_ascii($domain);
 		}
 
 		return (bool) filter_var($str, FILTER_VALIDATE_EMAIL);
diff --git a/user_guide_src/source/changelog.rst b/user_guide_src/source/changelog.rst
index aa9af21..5bdcda9 100644
--- a/user_guide_src/source/changelog.rst
+++ b/user_guide_src/source/changelog.rst
@@ -23,6 +23,7 @@
 -  Fixed a bug (#4762) - :doc:`Cache Library <libraries/caching>` 'file' driver method ``get_metadata()`` checked TTL time against ``mtime`` instead of the cache item's creation time.
 -  Fixed a bug where :doc:`File Uploading Library <libraries/file_uploading>` generated error messages on PHP 7.1.
 -  Fixed a bug (#4780) - :doc:`compatibility function <general/compatibility_functions>` ``hex2bin()`` didn't reject inputs of type "resource".
+-  Fixed a bug (#4787) - :doc:`Form Validation Library <libraries/form_validation>` method ``valid_email()`` triggered ``E_WARNING`` when input emails have empty domain names.
 
 Version 3.1.0
 =============