Xsl:variable content should be empty

I'm a relative newcomer to xslt and I wonder if someone could kindly
explain to me what is the issue with the following use of xsl:variable
and how I can correct it:

      <xsl:for-each select="/fieldguide/account">
            
      <xsl:variable name="link" select="file_link"> </xsl:variable>
	.
	.
	<td><a target = "_parent" href="{$link}"><xsl:value-of
select="species_name"/></a></td>
	.
	.
	</xsl:for-each>

Here is a fragment of the XML:

<fieldguide>
  <account> 
     <species_name>Octodiceras fontanum</species_name>
     <file_link>../accounts/mosses/Octodiceras
fontanum_DMT.pdf</file_link>
     .
     .	
  </account>
</fieldguide>


As you can see, I am trying to construct an html link from the filename
and path held in the <file_link> element.

This works fine when I call the stylesheet in-line or from Javascript in
Internet Explorer, but when I run it with the PHP XSL extension, it
generates a warning: Xsl:variable content should be empty since select
is present. It's only a warning and the output is still as expected, but
clearly I'm doing something wrong.