pingback returns faultCode 0, no message

For some reason, the default filter attached to pingback errors, will not send an error message unless the error code is 48. From wp-includes/commment.php:

function xmlrpc_pingback_error( $ixr_error ) {
    if ( $ixr_error->code === 48 )
        return $ixr_error;
    return new IXR_Error( 0, '' );
}