widget control form problem: works but then breaks after save

Hi, I am a little new to authoring widgets. I am working on a widget that displays images from a category you select. I have a dropdown list of categories. the problem I am having is that when you load the widget control panel, the dropdown is functional; it displays properly and you can sucessfully select a category. when you hit save, it does in fact save your selection, but then the dropdown list becomes an empty dropown with a single blank item. When you refresh, it loads properly again. I have no idea what is going on! Here is that chunk of my widget form code:

<p>
  <label for="<?php echo $this->get_field_id('whichCat'); ?>">
  <select id="<?php echo $this->get_field_id('whichCat'); ?>" name="<?php echo $this->get_field_name('whichCat'); ?>" />

<?php
  	//lets try and list some categories
 $categories = get_categories();
foreach ($categories as $cat) {
//set up the option in the drop down
echo "<option value=".$cat->slug.">";
echo $cat->name."</option>";
}
?>
</select></label></p>

thanks!