Making a script that makes rocks fall on a player?

So, when I put this code in, the output box just says "argument 1 missing or nil" at line 15 and I'm quite positive that there could possible be more errors. I personally Don't get what it's coming from, but here's my script. It makes 20 rocks spawn over the head of all players.

rock = script.Parent.Parent.Parent.rocky.Rock
for i, v in pairs(game.Players:GetChildren()) do
    getp = Instance.new("StringValue")
    getp.Value = tostring(v.Name)
    getp.Parent = game.Workspace
end


for i = 1,20 do
    local p = rock:Clone()
    p.Parent = script.Parent.Parent.Parent
    for i, v in pairs(game.Workspace:GetChildren()) do
        if v:IsA("StringValue") then
            local gname = v.Value
            local pl = game.Workspace:FindFirstChild(gname.Value)
            p.Position = pl.Position + Vector3.new(0,40,0)
            p.Anchored = false
        end
    end
end