xsl:key function help

Dear all,

as a newbie I tried to play with the key function but didnt get it. Could
someone help me out?

I got following stylesheet just to test wether its working or not:

<?xml version='1.0' encoding='UTF-8'?>
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:key name="Categories" match="Tip" use="@Topic" />
<xsl:output method="xml"/>
	
	<xsl:template match="/">
		<xsl:value-of select="count(key('Categories',Topic))"/>
	</xsl:template>

</xsl:stylesheet>

The Xml is:

<TipDatabase>
	<Tip Topic="Microsoft" Subtopic="Exchange">
		<Title>
			Make public folders available offline
		</Title>
		<Content>
			Public folders can be made available offline by
right clicking the folder and adding it to the Favorites folder.
			<Image Name="AddPublicFoldersToFavorites.jpg"/>
			Then adding them to the list of synchronized folders
by using keyboard combination Ctrl+Alt+S and selecting Edit.
			<Image Name="PublicFoldersOfflineAvailable.jpg"/>
		</Content>
	</Tip>
...
<TipDatabase>

Basically I want to use Muenchian Grouping to render my navigation later but
first I have to get the concepts clear in my head.

I use the stylesheet inside .net transform method. When I output the above
stylesheet The result is:

0

What is wrong here or what am I missing.
David