I have a content area that uses an interface to restrict that content can be added to it. The restriction is to a marker interface that's defined on both a page type and a block type. There no documentation that describes any restrictions to what the interface has to implement in the documentation as far as I can tell:
However, whe I search for the error I'm getting I found a thread where Casper Rasmussen say that the interface has to implement IContent. If that's the case, it should be mentioned in the documentation (the page above). However - a quite common case is that you want to allow either a page or a block that implements a given interface. Since the code for blocks don't implement IContent (this is implemented in the wrapping instance in Episerver) I don't know if this is possible.
I should mention that the content area works in general - you can always drag and drop content into it - but since I got a test report of this error I thought that I should investigate it.
A bug has been reported CMS-15794
I can add that the interface needs to implement IContentData to be able to handle both pages as well as shared blocks.