Handling GameTooltip

[Woei]

Just for testing, I'm trying to get GameTooltip to show information about an item I specify. So I've made a Button like the ones in your bags, and then I use this code to show the item information:

(in the XML)
<OnEnter>
TestMod_OnEnter();
</OnEnter>

(LUA)
function TestMod_OnEnter()
local tooltip = GameTooltip;
tooltip:SetOwner(this, "ANCHOR_RIGHT");
tooltip:SetBagItem(-1, 1); -- just an item in the bank
end
However, the tooltip doesn't show up at all. I'm using Auctioneer, and when i mouse over the button, I can see the Auctioneer tooltip has changed, the default tooltip doesn't show up at all though.
I've been searching the Internet and forums, but I couldn't find a lot about this subject (except for WoWWiki, but that's mainly about a scanning tooltip).
I know tooltips are difficult to handle, and I'm not very experienced in all this yet, but does anyone have a solution, or an example how to use tooltips correctly?

[Duugu]

function TestMod_OnEnter()
local tooltip = getglobal(GameTooltip);
tooltip:SetOwner(this, "ANCHOR_RIGHT");
tooltip:SetBagItem(-1, 1); -- just an item in the bank
end