custom tag template

Help!

I'm using the Nelo theme but wanted to create a custom tag template which shows all the tags within a specified category.

I have the following code in my tag.php (with hardcoded category and tag for testing), but it doesn't return anything. What am I doing wrong?

query_posts('category_name="July 2009"&tag="on-the-bookshelf"&showposts=5');

Either of these 2 simplified variations works fine:
query_posts('tag="on-the-bookshelf"&showposts=5');query_posts('category_name="July 2009"&showposts=5');

From what I read, the query_posts function should be able to do this.