[Typo3] Default (error) Page Not Set Correctly

[Rodrigo Cea]

Listies,

I have a site set up with typo3.6.2 and RealURL. Everything is just 
spiffy, except for the fact that a "bad" url 
(e.g.:"www.mydomain.com/thisisnotavalidpath") opens up an arbitrary 
page, not the site's root page. Is there some configuration, in RealURl 
or elsewhere, that says "if the path is wrong, open up PID X"?

Thanks,

Rodrigo