REST with $filter for List Items in Sharepoint 2013

Sharepoint 2013, REST API, native C++ application: I want to get all Folder items from the list. Try

/_api/Web/Lists(guid'GUID')/Items?$filter=FileSystemObjectType+eq+1

and get HTTP 400 Bed request. Same for $filter=AuthorId+eq+1 and some other fields.

But e.g. $filter=Id+eq+1 or $filter=Modified+eq+'2014-11-01T16:13:49Z' works fine.

Any ideas?