Count value not getting displayed for String colum

Hi,

   I am trying to display the count of the number of rows above the current row. The column is of type String. I have set a formula expression using the propertyExpression element, as shown below:

            <textField isStretchWithOverflow="true" isBlankWhenNull="true">
            <reportElement style="styleGroup" mode="Opaque" x="563" y="0" width="120" height="43">
   <propertyExpression name="net.sf.jasperreports.export.xls.formula"><![CDATA["COUNT(E1:E7)"]]></propertyExpression>
            </reportElement>
            <box>
              <topPen lineWidth="0.25"/>
              <leftPen lineWidth="0.25"/>
              <bottomPen lineWidth="0.25"/>
              <rightPen lineWidth="0.25"/>
            </box>
            <textElement textAlignment="Center" verticalAlignment="Middle" lineSpacing="Single"/>
            <textFieldExpression class="java.lang.String"/>
          </textField>

  I expect the value to be diaplyed as 7, as there are 7 rows above the current row.The problem is that the value is getting displayed as 0.  If I use the same expression for a column of type Integer or Double, the correct value is displayed. I need to mandatorily use the FORMULA attribute for this case and cannot use the Group_Count or other internal variables provided by Jasper. Is it mandatory for the column to be of type Integer or Double for the FORMULA attribute to display the value?..

Thanks