Subquery in IN does not work

I'm using TransactSQL (Microsoft SQL Server 2014) and here's my problem: I'm using the standard "Northwind" database for practice and I wanted to see a list of Categories that don't have any Products. Firstly I created a new Category, which wouldn't have any Products: insert into Categories (CategoryName) values ('TestCategory') Then I wrote this: SELECT CategoryName FROM Categories WHERE CategoryID NOT IN (select CategoryID from Products) Unfortunately it gives nothing. Here's how I understand it: I want to find categories without products connected with them, so I used WHERE CategoryID NOT IN (select CategoryID from Products), because I want to compare every CategoryID from Categories table with a list which I get from select CategoryID from Products - this list includes every category that HAS products. So I think that using NOT IN should let me to see categories that don't have any products. I hope you understood what I was trying to describe. It would be great if you could help.