Index: src/wp-includes/user.php
===================================================================
--- src/wp-includes/user.php	(revision 29749)
+++ src/wp-includes/user.php	(working copy)
@@ -1679,7 +1679,8 @@
 	if ( empty( $userdata['user_nicename'] ) ) {
 		$user_nicename = sanitize_title( $user_login );
 	} else {
-		$user_nicename = $userdata['user_nicename'];
+		$sanitized_user_nicename = sanitize_user( $userdata['user_nicename'], true );
+		$user_nicename = sanitize_title( $sanitized_user_nicename );
 	}
 
 	// Store values to save in user meta.