Widget Logic – Newbie

I am trying to use Widget Logic to display a widget only on the posts of a certain category.

This is what I put into the Widget Logic Box:

is_single() && in_category('47')

It didn’t work.

I tried both the full name of the category, the id number, and the slug.

Did I use the wrong code?

I am using the classipress theme, and so tried checking the use wp_reset_query fix and also tried putting the following in my sidebar.php

<php wp_reset_query() ?>

What else can I do?