404 problems

[eluser]Kurai[/eluser]
Hi!
There seems to be a problem with 404 error pages on my codeigniter site. It seems not to get 404 error page correctly. When I try to access a nonexistent page, I get this message instead the default 404 CI page:

Not Found

The requested URL /wp-admin was not found on this server.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

What should the problem be? Thank you!