Get NaN when adding

["James Steven" <JSteven@xxxxxxxxxxxxxxxxxxxxx>]

Hello
Using the xml and xsl below I have added b/c/data to b/d/data.  However when
I use the same method to add b/e/data (value of 0.00) to b/f/data (value of
0.00 also) I get NaN.  Does anyone have any ideas why I am getting this?
Thankyou very much for any help offered.


<xsl:template match="a">
 <xsl:call-template name="orderlist.sum.edit">
  <xsl:with-param name="set-of-order" select="b/c"/>
 </xsl:call-template>
</xsl:template>

<!-- ================================================================ --!>

<xsl:template name="orderlist.sum.edit">
 <xsl:param name="set-of-order"/>
  <xsl:variable name="sumorg">
   <xsl:call-template name="orderlist.sum">
    <xsl:with-param name="set-of-order" select="$set-of-order"/>
   </xsl:call-template>
  </xsl:variable>
 <xsl:value-of select='format-number($sumorg, "£###,###,##0.00")'/>
</xsl:template>

<xsl:template name="orderlist.sum">
  <xsl:param name="set-of-order"/>
  <xsl:variable name="var1">
   <xsl:call-template name="orderlist.sum.edit2">
    <xsl:with-param name="set-of-order" select="b/d"/>
   </xsl:call-template>
  </xsl:variable>
  <xsl:choose>
   <xsl:when test="$set-of-order">
   <xsl:variable name="first">
   <xsl:apply-templates select="$set-of-order[1]/data"/>
   </xsl:variable>
   <xsl:variable name="rest">
   <xsl:call-template name="orderlist.sum">
    <xsl:with-param name="set-of-order" select="$set-of-order[position() !=
1]"/>
   </xsl:call-template>
  </xsl:variable>
  <xsl:value-of select="$first + $rest + $var1"/>
 </xsl:when>
 <xsl:otherwise>0</xsl:otherwise>
 </xsl:choose>
</xsl:template>

<!-- ================================================================ --!>

<a>
 <b>
  <c>
    <data>1</data>
  </c>
   <c>
    <data>3</data>
  </c>
  <d>
    <data>1</data>
  </d>
  <d>
   <data>5</data>
  </d>
  <d>
   <data>2</data>
  </d>
  <e>
   <data>0.00</data>
  </e>
  <e>
   <data>0.00</data>
  </e>
  <f>
   <data>0.00</data>
  </f>
  <f>
   <data>0.00</data>
  </f>
 </b>
</a>