Why OnTooltipSetUnit callback is not working?

[nyxxir]

GameTooltip:HookScript("OnTooltipSetUnit", TooltipCallback)
 
function TooltipCallback(tooltip, ...)
    local unitName, unitId = tooltip:GetUnit()
    GameTooltip:AddLine(unitName)
end

The new line is not added. What is wrong? And what arguments OnTooltipSetUnit callback should have? I've a lot of different options ('self', 'self, tooltip', 'self, ...', 'tooltip, ...' and so on)...