Cannot modify header information - headers already sent by (output started at \wp-includes\functions.php:4986) in \wp-includes\pluggable.php

Cannot modify header information - headers already sent by (output started at \wp-includes\functions.php:4986) in \wp-includes\pluggable.php on line 1265

В ошибке указываются два файла ядра wordpress, получается ошибка в ядре?