Modx error page won't display

I've developed a site in modx but after migrating it to the live server if I enter a page that doesn't exist (eg http://mysite.com/index.php?id=5) I'm getting the default IIS error page rather than the page specifie开发者_开发知识库d in Tools->Configuration->site->Error page

Any idea why this could be happening?

For the record I developed on Apache but am deploying to IIS.

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The only thing I can think of is that IIS is over-riding the modX error page somehow.

I know on Apache I can fiddle with the .htaccess file to set ErrorDocument, but I don't know how IIS works.

Perhaps previous SO questions on "custom 404 page on IIS" might give you some clues as to how your system is setup.