304 response does not set custom header for apache with mod_headers

Question

<VirtualHost *:80>
    ServerAdmin webmaster@dev.dom.com
    DocumentRoot "C:/Program Files/Apache Software Foundation/Apache2.2/htdocs"
    ServerName dev.dom.com
    ServerAlias dev.dom.com
    ErrorLog "logs/dev.dom.com-error.log"
    CustomLog "logs/dev.dom.com-access.log" common
    PassEnv CLUSTER
    Header always set X-Cluster "%{CLUSTER}e"
</VirtualHost>

Here is my configuration. I have an environment variable which tells me what cluster I am on, which is passed as a header in 'X-Cluster'. This returns fine on a 200 or a 404 response, but a 304 Not Modified response never returns the header, even though it returns other appropriate Apache headers.

How do I get the header to be set during a 304 response?