Custom Error Documents for Subdomains

[Jack Timber]

I've modified the html pages in error_documents folder that comes as default. It's working fine with the main website, I can see those html pages being displayed in case of errors but when I create a subdomain then a default error message is being shown from IIS (html pages from error_documents are being used).

Should I have a separate version of error documents for every subdomain ?
How can I use the same error_documents for all subdomains ?

My directory structure:

Hosting Account Root
|-- error_documents
|-- (httpdocs) Main Website
|-- (Subdomain 1)
|-- (Subdomain 2)

I'm on Plesk 12.0 environment.