how to count nodes

hi,

I'd like to know the number of nodes for a key I have defined.  Im not getting the correct output, so I thought I could count the nodes something like this:

<xsl:value-ofselect="count(key('groups',Groupnum))"/>

But, I get all zeroes.  Is this the correct syntax?  

This is the key I defined :

<xsl:key name="groups" match="Row" use="Groupnum"/>  <!-- key value -->

Thank you

mhoran