diff --git a/wp-admin/includes/user.php b/wp-admin/includes/user.php
index 1d132b372f..bb68ad9c62 100644
--- a/wp-admin/includes/user.php
+++ b/wp-admin/includes/user.php
@@ -143,7 +143,9 @@ function edit_user( $user_id = 0 ) {
 		$errors->add( 'user_login', __( '<strong>ERROR</strong>: This username is already registered. Please choose another one.' ));
 
 	/** This filter is documented in wp-includes/user-functions.php */
-	if ( in_array( $user->user_login, apply_filters( 'illegal_user_logins', array() ) ) ) {
+	$illegal_logins = apply_filters( 'illegal_user_logins', array() );
+
+	if ( in_array( strtolower( $user->user_login ), array_map( 'strtolower', $illegal_logins ) ) ) {
 		$errors->add( 'illegal_user_login', __( '<strong>ERROR</strong>: Sorry, that username is not allowed.' ) );
 	}
 
diff --git a/wp-includes/ms-functions.php b/wp-includes/ms-functions.php
index 62a2d4e37a..d54a838e10 100644
--- a/wp-includes/ms-functions.php
+++ b/wp-includes/ms-functions.php
@@ -432,7 +432,9 @@ function wpmu_validate_user_signup($user_name, $user_email) {
 	}
 
 	/** This filter is documented in wp-includes/user-functions.php */
-	if ( in_array( $user_name, apply_filters( 'illegal_user_logins', array() ) ) ) {
+	$illegal_logins = apply_filters( 'illegal_user_logins', array() );
+
+	if ( in_array( strtolower( $user_name ), array_map( 'strtolower', $illegal_logins ) ) ) {
 		$errors->add( 'user_name',  __( 'Sorry, that username is not allowed.' ) );
 	}
 
diff --git a/wp-includes/user-functions.php b/wp-includes/user-functions.php
index 89ee9c8574..1a71fcaaad 100644
--- a/wp-includes/user-functions.php
+++ b/wp-includes/user-functions.php
@@ -1328,7 +1328,9 @@ function wp_insert_user( $userdata ) {
 	 *
 	 * @param array $usernames Array of blacklisted usernames.
 	 */
-	if ( in_array( $user_login, apply_filters( 'illegal_user_logins', array() ) ) ) {
+	$illegal_logins = apply_filters( 'illegal_user_logins', array() );
+
+	if ( in_array( strtolower( $user_login ), array_map( 'strtolower', $illegal_logins ) ) ) {
 		return new WP_Error( 'illegal_user_login', __( 'Sorry, that username is not allowed.' ) );
 	}
 
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 064396a80f..ecbc055408 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.4-beta4-35628';
+$wp_version = '4.4-beta4-35629';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.