customerrors in subdirectory

I have a subdirectory in asp.net web site where exists web.config with customerrors element.
Also in root application directory there is web.config with customerrors.
Both customerrors have custom error pages.
I have turned both customerrors on, but I don't get custom error page from subdirectory when I throw exception from page in subdirectory.
Instead it gives me error message that I need to turn RemoteOnly.
What is the problem, how to make it work?
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