String:sub not working?

I made a part of a function that will kill someone if they do not have the name of the third letter and above of a model. The model in question would be called TB[Name], but this script kills everyone regardless of their name. Here is the script:

local nm = hit.Parent.Parent.Name
        local tb = script.Parent.Parent.Name
        if nm ~= tb:sub(3) then
        hit.Parent.Humanoid:TakeDamage(100)
        script.Parent.hit:Play()
        wait(1)
    end
    end

There are no errors in the output. Am I using :sub correctly?