SubHeading Plugin not outputing properly

I’m using the SubHeading plugin on my site and I’m using the following code in my single.php template:
<?php the_subheading(': <span>','</span>') ?>

However, when I view a single post without a subheading being used, it still displays the : <span></span>

Is there a way to not display the subheading before & after text when there is no subheading on the post?

http://wordpress.org/extend/plugins/subheading/