[Zope] set Expires header in ZPT

[Sakesun Roykiattisak]

I've tried this in ZPT:

<span tal:define="dummy python:request.response.setHeader('Expires', 0);
                  dummy python:request.response.setHeader('test', 'test')">
  <h1 tal:content="python:request.response.getHeader('test')"/>
  <h1 tal:content="python:request.response.getHeader('a')"/>
</span>


It happens that only the "test" header is displayed, the "Expires" 
header is not.
Does Zope do somethine special with "Expires" header ?  How can I set it 
then ? Through property ?