[Zope] Error Value: sequence-var-get_size

[Zoper]

Hi All!

How can I use the sequence-var-get_size variable?
I have the following DTML Method:

    <dtml-var standard_html_header>
    <table cellpadding="5" cellspacing="0" border="0">
    <tr bgcolor="#CFCFCF">
    <td><b>Type</b></td>
    <td><b>Name</b></td>
    <td><b>Size</b></td>
    <td><b>Last Modified</b></td>
    </tr>
    <dtml-in expr="objectValues()" sort="id" >
    <dtml-if sequence-even>
    <tr bgcolor="#DCDCDC">
    <dtml-else>
    <tr>
    </dtml-if>
    <td><img src="<dtml-var sequence-var-icon>" border="0"></td> <td><a
    href="&dtml-absolute_url;"><dtml-var sequence-var-id></td> <td>
    <dtml-var sequence-var-get_size> bytes</td> <td>
    <dtml-var sequence-var-bobobase_modification_time></td>
    </tr>
    </dtml-in>
    </table>
    <dtml-var standard_html_footer>

but whenever I try to run it I get an ugly Zope message:
        "This resource may be trying to reference a nonexistent object or
variable sequence-var-get_size".
What is the problem?

Best Regards.

Giuseppe.