[kurapica.igas]

When use a frame to register non-existed event, it'd throw an error, that's why the first try failed.

I guess the AceEvents handle it like

local events = {}
local frame = CreateFrame("Frame")
 
frame:SetScript("OnEvent", function(self, event, ...)
    for frm, handler in pairs(events[event]) do
        frm[handler](frm, event, ...)
    end
end)
 
function addon:RegisterEvent(event, handler)
    if not events[event] then
        events[event] = {}
 
        -- Where the error raised when event not existed
        -- But the events[event] already existed, so no error next time
        frame:RegisterEvent(event)
    end
 
    -- register the handler to the addon
    events[event][self] = handler
    
    return true
end

So, you can make a version of your own, it's not complicated