I think it's a bug, because I tried this on a brand new django project and created page model as described in documentation. Then, I registered the only 'relatedpages' extension, and got this error.

'tuple' object has no attribute 'extend'

(dpaste)

Also, if I try to register datepublisher together with relatedpages, I get

There are duplicate field(s) in PageAdmin.fieldsets

( dpaste)