[users@httpd] 404 on existing file

[Liz Kim]

I am having a problem with ErrorDocument.
There is a err.html file which exists under
/web/path/to/err.html

ErrorDocument 500 http://myURL/path/to/err.html

works.  However,

ErrorDocument 404 /web/path/to/err.html

gives me a 404 error.  The file exists and I can get to it
when I give it the complete url.  

Any inputs on why it's giving me the 404 on the local URL?

Thank you