When the ISAPI redirector processes a response to a 304 (or 204/205) response with chunked encoding enabled, and a Content-Length is not specified, chunked encoding is applied to the empty body, which created an invalid response.

Under normal operation (e.g. DefaultServlet responses), the closing of the output buffer writes a Content-Length: 0 to the response, which the AJP connectors don't try to remove for 304 etc. responses (the HTTP connectors do). This results in 304 responses with Content-Length: 0 headers, which is valid (if quirky).

This issue can be reproduced by setting a 304 response status and explicitly calling response.flushBuffer() to commit the response prior to closing the output buffer.

The ISAPI Redirector also isn't handling a similar case for HEAD requests.