passing element to function having an 'xs:anyAtomicTyp

<xsl:stylesheet
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  version="2.0"
  xmlns:xs="http://www.w3.org/2001/XMLSchema"
  xmlns:functx="http://www.functx.com">

  <xsl:function name="functx:day-of-week" as="xs:integer?">
    <xsl:param name="date" as="xs:anyAtomicType?"/>

    <xsl:sequence select="
    if (empty($date))
    then ()
    else xs:integer((xs:date($date) - xs:date('1901-01-06'))
            div xs:dayTimeDuration('P1D')) mod 7
   "/>

  <xsl:template match="/">
    <xsl:value-of
      select="root/date-list/date/functx:day-of-week(.)"
      separator="&#13;&#10;"/>
  </xsl:template>

<root>
  <date-list>
    <date>2008-02-18</date>
    <date>2008-02-19</date>
    <date>2008-02-20</date>
    <date>2008-02-21</date>
    <date>2008-02-22</date>
    <date>2008-02-23</date>
    <date>2008-02-24</date>
  </date-list>
</root>

Error in XPath 2.0 expression at as
C:\path\sheet.xml
Line 10, Character 29