diff --git a/system/libraries/Validation.php b/system/libraries/Validation.php
index 16e8cd7..f628d86 100644
--- a/system/libraries/Validation.php
+++ b/system/libraries/Validation.php
@@ -117,7 +117,7 @@
 			if ($rules == '')
 				return;
 				
-			$data[$data] = $rules;
+			$data = array($data => $rules);
 		}
 	
 		foreach ($data as $key => $val)
diff --git a/user_guide/changelog.html b/user_guide/changelog.html
index ea14a39..6521636 100644
--- a/user_guide/changelog.html
+++ b/user_guide/changelog.html
@@ -116,6 +116,7 @@
 	<li>Fixed an AR_caching error where it wasn't tracking table aliases (#3463).</li>
 	<li>Fixed an AR bug with or_where_not_in() (#4171).</li>
 	<li>Fixed a bug in the FTP library where delete_dir() was not working recursively (#4215).</li>
+	<li>Fixed a Validation bug when set_rules() is used with a non-array field name and rule (#4220).</li>
 	<li>Fixed an AR bug when joining with a table alias and table prefix (#4400).</li>
 	<li>Fixed a bug in DB Forge, when inserting an id field (#3456).</li>
 	<li>Fixed a bug in the DB class testing the $params argument.</li>