setText or setFormattedText or ?

[Lazare]

I have a button defined in my xml file as TRACK_QUEST_ABBREV The normal text for it is "Track". I would love to be able to change it to "UnTrack" if the quest is being tracked (like the Blizz quest log does). I copied the following Lua code from the Blizz quest log but it is giving me errors:

function qgc:ToggleWatch(index)
    if not index then
        index = GetQuestLogSelection()
    end
    if index>0 then
        if IsQuestWatched(index) then -- already watched, remove from watch
            RemoveQuestWatch(index)
            TRACK_QUEST_ABBREV:SetText(TEXT("Track"))
        else -- not watched, see if there's room to add, add if so
            if GetNumQuestWatches() >= MAX_WATCHABLE_QUESTS then
                UIErrorsFrame:AddMessage(format(QUEST_WATCH_TOO_MANY,MAX_WATCHABLE_QUESTS),1,0.1,0.1,1)
            else
                AddQuestWatch(index)
                TRACK_QUEST_ABBREV:SetText(TEXT("UnTrack"))
            end
        end
    end
end

What am I missing? The error that I get is: attempt to call method 'SetText' (a nil value) and SetFormattedText gets the very same error.
Thanks in advance for any help on this.
Cheers!