[Zope] Strange CSS inclusion BUG

[Li Dongfeng]

To use CSS file, I put the following line in the head of HTML files:

    <dtml-if local_css>
    <link href="<dtml-var "local_css.absolute_url()">"
          rel="stylesheet" type="text/css">
    </dtml-if>

Where "local_css" is a DTML document containing CSS style sheet.
But the DTML generates the following error message:

    Error Type: AttributeError
    Error Value: 'string' object has no attribute 'absolute_url'

If I strip the <dtml-if  protection, the code works OK.
Why is the DTML document local_css changed into a string 
after <dtml-if>?