RSS Language missing

My RSS feed suddenly stopped validating - with:

Sorry

This feed does not validate.

    line 16, column 11: language must be an ISO-639 language code: [help]

Looking at the PHP file I see that the language is worked out thus:

<language><?php echo get_option('rss_language'); ?></language>

if you change it to :

<language><?php echo bloginfo_rss( 'language' ); ?></language>

it starts validating again.