Minimap Scroll not working after 4.0.1

[sacrife]

This worked fine before, now it does not.
I know that I have to replace arg1 with something, but I really do not know what? Can anyone point me in the right direction here?

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-- Zoom
----------------------------------
Minimap:EnableMouseWheel(true)
Minimap:SetScript('OnMouseWheel', function()
    if (arg1 > 0) then
        Minimap_ZoomIn()
    else
        Minimap_ZoomOut()
    end
end)