Fatal error: call to undefined function Is_tag()

Hi all, I am using the magazine theme named Mombi, v2.2, but when I try to access the category or monthly archives, an error “call to undefined function is_tag()” occured.

I check the line where error occured, here is part of it:

<?php is_tag(); ?>
<?php if (have_posts()) : ?>

<?php $post = $posts[0]; // Hack. Set $post so that the_date() works. ?>
<?php /* If this is a category archive */ if (is_category()) { ?>
<h2 class=”pagetitle”>Archives for the ‘<?php single_cat_title(); ?>’ Category</h2>

<?php } ?>

and my WordPress version is 2.3.3. Please help me.