stop apache injecting it's own 404 not found page to the custom 404 error page

Question

I'm having a problem implementing custom 404 error pages. my setup is a front-end apache 2.2.6 proxy request other than static file to the backend app server(apache 1.3.36)

i send 404 header back along with error page when user request a non-existing page to the backend server. I do get the error page back but somehow the backend apache server injects its own 404 not found page in the error page as well.

here is what injected to the not found page by apache:

Not Found
The requested URL /product/8jd4776 was not found on this server.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.
Apache/1.3.36 Server at www.example.com Port 8888 

any idea why this is happening?

Answer 1

The content length of your custom 404 message must be 512 bytes or more.

Answer 2

set "ProxyErrorOverride On" with ErrorDocument in the front-end apache server solve the problem.