diff --git a/wp-includes/ms-functions.php b/wp-includes/ms-functions.php
index 1e10e4abf..b75ed7eee 100644
--- a/wp-includes/ms-functions.php
+++ b/wp-includes/ms-functions.php
@@ -423,7 +423,7 @@ function wpmu_validate_user_signup($user_name, $user_email) {
 	preg_match( '/[a-z0-9]+/', $user_name, $maybe );
 
 	if ( $user_name != $orig_username || $user_name != $maybe[0] ) {
-		$errors->add( 'user_name', __( 'Only lowercase letters and numbers are allowed.' ) );
+		$errors->add( 'user_name', __( 'Only lowercase letters (a-z) and numbers are allowed.' ) );
 		$user_name = $orig_username;
 	}