Interfaces that use a Struct type end up with an assertion failure in libdbus:
arguments to dbus_message_iter_open_container() were incorrect, assertion "(type == DBUS_TYPE_STRUCT && contained_signature == NULL) || (type == DBUS_TYPE_DICT_ENTRY && contained_signature == NULL) || (type == DBUS_TYPE_VARIANT && contained_signature != NULL) || (type == DBUS_TYPE_ARRAY && contained_signature != NULL)" failed in file dbus-message.c line 2949.
The attached patch solves this.
This seems to be a duplicate of bug #5.