Inherited method in PyROOT/RooFit not found

Using root6 and PyROOT to add a RooArgSet to another one, I notice the following behaviour:

import ROOT

set1 = ROOT.RooArgSet()

set2 = ROOT.RooArgSet()

set1.add(set2)

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: bool RooArgSet::add(const RooAbsArg& var, bool silent = kFALSE) =>
could not convert argument 1

even though

RooArgSet inherits the function Bool_t RooAbsCollection::add (const RooAbsCollection& list, Bool_t silent = kFALSE)

Doing the same in an interactive ROOT session behaves as expected:

RooArgSet set1;

RooArgSet set2;

set1.add(set2);

(Bool_t) false

With root5 and PyROOT the behaviour is as expected naively:

import ROOT

set1 = ROOT.RooArgSet()

set2 = ROOT.RooArgSet()

set1.add(set2)

False