Comparing nodes in XPath

Why is this xpath NOT working

<xsl:value-of select="document('connections.xml')/connections/connection[@id=Conn]"/>

 and this IS

<xsl:variable name="conn" select="Conn"/>
<xsl:value-of select="document('connections.xml')/connections/connection[@id=$conn]"/>

I can think that the first is comparing a attribute to a node so it
allways returns a node set(0)?

And the second is comparing a attribute with a string?

I also tryed using text(Conn), Conn/text() and some others, but none work,


This is probably a basic question, maybe it's getting late for me...