Error in watchdog displaying nodes with comments

I am getting the following in the watchdog log, viewing any node with comments:

Missing argument 2 for variable_get(), called in /var/www/xxx.com/sites/xxx.com/themes/barlow/comment.tpl.php on line 9 and defined in /var/www/xxx.com/includes/bootstrap.inc on line 502.

The line in question reads:

<span class="author"><?php print $author; ?></span><?php if (variable_get('comment_subject_field_' . $node->type)): ?>

If I change the code to read:

<span class="author"><?php print $author; ?></span><?php if (variable_get('comment_subject_field_' . $node->type,1)): ?>

The error goes away. I have no idea what I'm doing or what the second arguement should be though, but it appears to correct the error as a dumb i-dont-understand-php fix.

Using:
Drupal 6.16
Barlow 1.1
PHP 5.2.6

Happy to provide any other information required.

Tim