param weirdness

This *should be* obvious, but sometimes the obvious isn't always to this
observer.  :)

    <xsl:call-template name="detail">
       <xsl:with-param name="category">markup</xsl:with-param>
    </xsl:call-template>

...

    <xsl:template name="detail">
      <xsl:param name="category" />
        <xsl:for-each select='/config/option[@class="$category"]'>
          ...
        </xsl:for-each>
    </xsl:template>

The for-each selects *nothing*, but if I change "$category" to the
hard-coded string "markup", it works fine!

The `$category` parameter does come through, or so 

    <xsl:value-of select='$category' />

tells me.  So why does does a hard-coded category work, while "$category"
fails?


/Jelks