404 Custom Error page not displaying for unknown .aspx page

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hi,

I have made a custom page for 404 error.
Everything seems to be correct like SEO Headers and redirecttion to this custom 404 page.

Except that if i type the url and /123.aspx it gives some XML parser error.
But other extensions and without any extensions works fine.

So whats the problem with the unknown .aspx extension.

Please reply its bit urgent