Using screen function in hs.layout results in "Unable to find display: function: 0x60800084a3b0"

error.lua

layout = {
    {"Mail", nil, function() attemptSecondaryScreen() end, hs.geometry.unitrect(0, 0, 1, 0.7), nil, nil},
    {"Slack", nil, attemptSecondaryScreen, hs.geometry.unitrect(0, 0.3, 1, 0.7), nil, nil}
}

function attemptSecondaryScreen()
    local screens = hs.screen.allScreens()
    if #screens > 1 then
        for _, scr in ipairs(screens) do
            if scr ~= hs.screen.primaryScreen() then
                return scr
            end
        end
    end
    return hs.screen.primaryScreen()
end

Answer: attemptSecondaryScreen() must appear as a call in the definition. Not as a reference to a function.