xsl compare count of elements to 0

I'm trying to compare the count of an list of elements to 0, this is how I get the elements:

<xsl:variable name="elments" select="/Elements/ElementList[@IstImportant='true']"></xsl:variable>

and this is how I try to compare the count of the elements:

<xsl:choose>
   <xsl:when test="count($elments) &gt; 0">
       <xsl:attribute name="Num">1500</xsl:attribute>
    </xsl:when>
    <xsl:otherwise>
       <xsl:attribute name="Num">1501</xsl:attribute>
    </xsl:otherwise>
</xsl:choose>

But I get allways "1501" as the num attribute, even if the count of elements with IstImportant='true' is greater than 0. What have I done wrong.