Not Found Directed to Index

[Todd Orr]

I've setup a custom 404 page according to
http://wiki.apache.org/tapestry/Tapestry5HowToUseTapestryForCustomErrorPages.
However, whenever I navigate to a URL that does not exist Tapestry simply
renders the index page for that directory or a directory above if none
exists in that directory. How do you force Tapestry to use a custom 404?