Wordpress - Error "Trying to get property of non-object in..." after calling function get_the_category();

I'm trying to get the respective category name a single portfolio post belongs to, and paste it into an anchor tag's href attribute. My current code looks like so:

$category = get_the_category($post->ID);

//And little further down the php file...

<a href=" <?php echo get_site_url() . '/' . $category[0]->cat_name) . '/' ?>" />

I've referred to the documentation several times now, and I'd like to know some of your suggestions as my attempts to achieve what I want have ended in little success.