Function is screwing with message:Sub()?

Alright, this script takes a target of a string and prints it out one by one, however, I'm getting an error and I'm not sure why.

Not working

MainDest = script.Parent.speechLabel

String = "Why this no work.."

function Speak(message, target)
    for i = 1, message:len() do
        wait()
        target.Text = message:Sub(1,i)
    end
end

wait(1)
Speak(String, MainDest)

Error:

8: attempt to call method 'Sub' (a nil value)

Working without function

for i = 1, String:len() do
wait()
Message.Text = String:sub(1,i)
end

Any help would be great! Thanks!