Search Box stops working when floated

I added a search box to my navigation bar using this code:

<div id="search-box">
<form method="get" id="searchform" action="<?php bloginfo('home'); ?>/">
<div><input type="text" size="18" value="<?php echo wp_specialchars($s, 1); ?>" name="s" id="s" />
<input type="submit" id="searchsubmit" value="Search" class="btn" />
</div>
</form>
</div> <!-- #search-box -->

The box worked fine, until I changed the positioning of the “search-box” div with the following code ( .custom is for Thesis ):

.custom #search-box {
	float:right;
	}

After I added the CSS, the search box still displayed as expected but stopped working. I tested removing the float and adding a background color to the div, and the search box started working again (see below) so it appears the positioning is the issue:

.custom #search-box {
	background-color:red;
	}

I’m stumped and appreciate any help! I can’t include a link to the site because it is in development on my local machine.