diff --git a/system/libraries/Form_validation.php b/system/libraries/Form_validation.php
index 36a8148..0251286 100644
--- a/system/libraries/Form_validation.php
+++ b/system/libraries/Form_validation.php
@@ -618,6 +618,12 @@
 					$rules = array(1 => $rule);
 					break;
 				}
+				elseif (is_array($rule) && isset($rule[0], $rule[1]) && is_callable($rule[1]))
+				{
+					$callback = TRUE;
+					$rules = array($rule[0] => $rule[1]);
+					break;
+				}
 			}
 
 			if ( ! $callback)
diff --git a/user_guide_src/source/changelog.rst b/user_guide_src/source/changelog.rst
index 3a6db3d..1d9fa0b 100644
--- a/user_guide_src/source/changelog.rst
+++ b/user_guide_src/source/changelog.rst
@@ -32,6 +32,7 @@
 -  Fixed a bug in the :doc:`Database Utility Class <database/utilities>` method ``csv_from_result()`` didn't work with a whitespace CSV delimiter.
 -  Fixed a bug (#3890) - :doc:`Input Library <libraries/input>` method ``get_request_header()`` treated header names as case-sensitive.
 -  Fixed a bug (#3903) - :doc:`Form Validation Library <libraries/form_validation>` ignored "unnamed" closure validation rules.
+-  Fixed a bug (#3904) - :doc:`Form Validation Library <libraries/form_validation>` ignored "named" callback rules when the field is empty and there's no 'required' rule.
 
 Version 3.0.0
 =============