Problem getting conditional statement to work in sidebar

I’ve got the following code which should be displaying a different block of code in my sidebar if the user is on the home page.

I’ve set up my front page to be a static page and to use a custom template. It’s doing all of that correctly, it’s just as if my sidebar isn’t seeing the page as the front page.:

<?php if (is_front_page() ) { ?>
<div id="sidebar-home" class="column span-5 last">
<p>this page IS the home page</p>
</div>

<?php } else { ?>
<div id="sidebar" class="column span-5 last" >
<p>this page isn't the home page</p>
</div>
<?php } ?>

It isn’t working, what am I missing.

thanks