Menu widget not showing on search.php page via get_header(); function

I have the following code in the header.php file which renders the main menu:

<?php if (function_exists('dynamic_sidebar') && dynamic_sidebar('Main Menu')) : else : ?>
      <p>main menu widget here</p>
<?php endif; ?>

I include it on pages and post pages via get_header() function, and it works fine there, but on the search results page, search.php the menu doesn’t show up (though the rest of the header file does show up).

Any ideas?