Sum produces NaN

Hello
To add a column of numbers I am using the xsl below.  However if there is
one value missing in the column then the xsl produces NaN.

eg. Below is xml with a value missing in last <data>.  I then use the xsl
below to add all <data> values but receive NaN because of the missing value.
If all <data> tags have value then the sum works.  Please could anyone show
how to resolve this.  Any help greatly appreciated.

<a>
  <b>
        <data>1</data>
  </b>
  <b>
      <data>2</data>
  </b>
  <b>
      <data>5</data>
  </b>
  <b>
      <data></data>
  </b>
</a>


<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: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"/>
                                        </xsl:when>
                                        <xsl:otherwise>0</xsl:otherwise>
                         </xsl:choose>
                  </xsl:template>