[Answered] Why is my table returning nil?

So I have a table and I want a random thing to be chosen from the table but every once in a while it chooses nil instead of a number to look for in the table.

chance = {1,1,1,1,1,1,1,1,1,2,2,2,2,2,2,3,3,3,4}
clen = 0
for k, v in pairs(chance) do
    clen = clen + v
end

while wait(30) do
    a = chance[math.random(1,clen-1)]
    warn(a) -- This sometimes returns nil instead of a number
    if a == 1 then
        newNormalBox(math.random(50,500))
    else if a == 2 then
        newGemBox(math.random(1,3))
    else if a == 3 then
        newGoldBox(math.random(2000,4000),math.random(1,10))
    else if a == 4 then
        newDiamondBox(math.random(20000,50000),math.random(5,40))
    end
    end
    end
    game.StarterGui:SetCore("SendNotification",{
        Title = "NOTICE"; 
        Text = "A box has spawned somewhere in the map. You should go see what's inside!";
        Icon = "";
        Duration =  5;
    })
    end
end