pingback returns faultCode 0, no message

I have implemented my own pingback client and sent out pingbacks to different hosts.

Some wordpress instances responded to the XML-RPC pingback request with a faultCode of 0 and an empty faultMessage.

What can be the cause for that?

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, '' );
}