How to set the members GiftedBadgeLookup in a community site?

I am trying to set the gifted badge of a community site member with the server object model. I have the badge id and title from the badges list and the member list item from the community member list.

SPFieldLookupValue giftedBadge = new SPFieldLookupValue(badgeId, badgeTitle);
memberItem["GiftedBadgeLookup"] = giftedBadge;
memberItem["GiftedBadgeText"] = badgeTitle;
memberItem.Update();

No exception, but the value of GiftedBadgeLookup is not changed. Any ideas?

Or do I've overlooked any APIs for the badge system?

Greetings
Peter