Automatic server shutdown when new version not working... Please help???

I'm trying to make a automatic shutdown when every there is a new version and this is the error I'm getting:

Stack Begin Workspace.Script:8: attempt to preform arithmetic on local 'len' (a string value) 'Workspace.Script', Line 8 - global Getversion 'Workspace.Script', Line 12 Stack End

Does anyone have any idea on what's going on??

mps = game:GetService('MarketplaceService')
_Version = nil

function GetVersion()
    local str = mps:GetProductInfo(game.PlaceId).Description
    print(str)
    local len = tostring(str:find('_Version'))
    return str.sub(len, len+5)
end

while wait(120) do
    local new w= GetVersion()
    if _Version then
        if _Version ~= new then
            local msg = Instance.new('Message', workspace)
            for i = 60,1 do
                msg.Text = 'The game is shutting down for an update in: ['..i..']'
            end
            while wait() do
                for i,v in pairs(game.Players:GetChildren()) do
                    if v:IsA'Player' then
                        v:Kick()
                    end
                end
            end
        end
    end
    _Version = new
end