Attempt to index nil with 'FindFirstChild'?

I'm trying to create a hold shift to sprint script. But when I try to get the player's character it gives the error that's in the title. Here's the code:

--Local script
local UIS = game:GetService("UserInputService")
local player = game.Players.LocalPlayer
local char = player.Character
local hum = player.Character:FindFirstChild("Humanoid")
local held = false


UIS.InputBegan:Connect(function(input, gameProcessedEvent)
    if input.KeyCode == Enum.KeyCode.LeftShift then
        held = true
    end
    while held == true do
        wait()
        hum.WalkSpeed = 60
    end
end)

UIS.InputEnded:Connect(function(input, gameProcessedEvent)
    if input.KeyCode == Enum.KeyCode.LeftShift then
        held = false
    end
end)