Does zero buffer allowed in the output buffer chain?

Hi,

I am writing a http handler module, I found when the last buffer in the output buffer chain is zero, I'll get an alert in the error log about zero output buffer, and the browser will get no response after waiting a while, this is not what I am expecting as the buffer chain has data, just the last one has zero size. Does this a feature or I missed something else?

PS:
nginx version: 1.2.4
gzip option is enabled.
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