diff --git a/src/wp-includes/ms-default-filters.php b/src/wp-includes/ms-default-filters.php
index c4c223a4f7..9d5da57f3e 100644
--- a/src/wp-includes/ms-default-filters.php
+++ b/src/wp-includes/ms-default-filters.php
@@ -18,7 +18,7 @@ add_filter( 'wpmu_validate_user_signup', 'signup_nonce_check' );
 add_action( 'init', 'maybe_add_existing_user_to_blog' );
 add_action( 'wpmu_new_user', 'newuser_notify_siteadmin' );
 add_action( 'wpmu_activate_user', 'add_new_user_to_blog', 10, 3 );
-add_action( 'sanitize_user', 'strtolower' );
+add_filter( 'sanitize_user', 'strtolower' );
 
 // Blogs
 add_filter( 'wpmu_validate_blog_signup', 'signup_nonce_check' );