Workflow to copy document from root site to sub-web?

Please help! I am trying to using Visual Studio 2012 to create a SP 2013 workflow that will copy a document library item to a document library in a sub web site.

Using the CopyItem activity I can sucessfully copy document items within the scope of the root site. I do this by supplying the destination library GUID and everything works fine.

If I supply the GUID of a doc library in a sub web site I recieve this error: {"error":{"code":"-2130575322, Microsoft.SharePoint.SPException","message":{"lang":"en-US","value":"List does not exist.\u000a\u000aThe page you selected contains a list that does not exist.  It may have been deleted by another user."}}}

Is it possible to target sub web site lists & libraries? If so how do I do this??

Thanks