No-protocol-handler-was-valid error

I am running a Mongrel cluster with an Apache front-end. I am loading
the following modules in the httpd.conf file:

LoadModule proxy_module modules/mod_proxy.so
LoadModule proxy_balancer_module modules/mod_proxy_balancer.so
LoadModule proxy_ftp_module modules/mod_proxy_ftp.so
LoadModule proxy_http_module modules/mod_proxy_http.so
LoadModule proxy_connect_module modules/mod_proxy_connect.so

Given those conditions, can anyone give me a reason why I would be
getting the the following error message when I attempt to access the
main/column page:

Forbidden
You don't have permission to access /main/column on this server.

The above error message is accompanied by the following entry in the
Apache error log:

[Wed Aug 29 02:19:53 2007] [warn] proxy: No protocol handler was valid
for the URL /main/column. If you are using a DSO version of mod_proxy,
make sure the proxy submodules are included in the configuration using
LoadModule.

Thanks for any input.

       ... doug