attempt to concatenate nil with string?

Attempt to concatenate nil with string here's the script

                                local handle =
             game.ReplicatedStorage:WaitForChild("ToolModels"):FindFirstChild(availableTools[1] 
            [i].."Handle").Clone() --Where I got the error
         handle.Parent = itemViewport

There is no variable stating what "i" is. Also, for the love of god please rearrange your code I've never seen anything this messy.