Ticket #24618: 24618.patch

wp-includes/user.php

@@ -1413 +1413 @@
  */
 function validate_username( $username ) {
         $sanitized = sanitize_user( $username, true );
-        $valid = ( $sanitized == $username );
+        $valid = ( $sanitized == $username && ! empty( $sanitized ) );
         /**
          * Filter whether the provided username is valid or not.
          *