WoWInterface - OnMouseUp and mouseover

I last created Mods back before Cataclysm. It's been a long time. Just now getting back into it. I'm just trying to write a very basic mod that cancels attacking with the right mouse button.

Code:

WorldFrame:HookScript("OnMouseUp", function(self, button)

    if button == "RightButton" then

        local mouseoverName = UnitName("mouseover") or "Unknown";

        local reaction = UnitReaction("mouseover", "player") or 10;

        if reaction < 5 then                        

            DEFAULT_CHAT_FRAME:AddMessage("Stopped Attack on: "..mouseoverName);

            MouselookStop();

        else

            DEFAULT_CHAT_FRAME:AddMessage(mouseoverName.." is friendly! "..reaction);

        end

    end

end)

The code I have should be working (from what I can tell) - but it's not. The code always returns Unknown & 10. If I swap out `mouseover` for `player` it works fine (but doesn't actually give me the data I need).

Also, if I swap out and hook-into `OnMouseDown` it also works, but then I can't actually interrupt the attack.

Any insight on why it's not working or how to fix it would be appreciated... Also if there is just a better way to do things I'm all ears.