qdbus asserts in qmetaobject when introspecting some services

Example:

[nemo@localhost ~]$ /usr/lib/qt5/bin/qdbus --system org.freedesktop.login1 /org/freedesktop/login1/user/100000
property read QString org.freedesktop.login1.User.DefaultControlGroup
ASSERT: "data.ptr->ref.isStatic()" in file kernel/qmetaobject.cpp, line 153

This has cropped up on other services also, for instance:

root@localhost:~% qdbus --system com.nokia.NonGraphicFeedback1.Backend
signal void com.nokia.NonGraphicFeedback1.Backend.Status(uint, uint)
method uint com.nokia.NonGraphicFeedback1.Backend.Pause(uint event_id, bool pause)
ASSERT: "data.ptr->ref.isStatic()" in file kernel/qmetaobject.cpp, line 153

valgrind does not report any warnings.