IsDescentOf is not working on my fireball script?

Basically everything works fine except that I am trying to make it to where when the player creates the fireball it does not damage them, it damages the other player.

19:48:52.920 - IsDescendentOf is not a valid member of Workspace

That is the error I get and here is my script.

local player = game.Players.LocalPlayer mouse = player:GetMouse()

mouse.KeyDown:connect(function(plr)

if plr == "q" then
	print(plr)
	
	
	local bob = Instance.new("Part")
	
	bob.Parent = game.Workspace
	bob.Shape = "Ball"
	bob.TopSurface = "Smooth"
	bob.BottomSurface = "Smooth"
	bob.Anchored = false
	bob.Size = Vector3.new(5,5,5)
	bob.CanCollide = false
	bob.Transparency = .5
	bob.BrickColor = BrickColor.new("Red")
	bob.CFrame = player.Character.Torso.CFrame *CFrame.new(0,0,-20)
	
	
	
	
	local thing = Instance.new("BodyVelocity",bob)
	thing.MaxForce = Vector3.new(math.huge,math.huge,math.huge)
	thing.velocity = player.Character.Torso.CFrame.lookVector*80
	
	
	
	
	
local chat = game:GetService("Chat")
chat:Chat(player.Character.Head, "FireBall")

	
	
	bob.Touched:connect(function(plr)
		local hum = game.Players.LocalPlayer.Character:FindFirstChild("Humanoid")
		if hum ~= nil and not bob.Parent:IsDescendentOf(script.Parent) then
			hum:TakeDamage(20)
		end
	end)
	
	
	
	
	local fire = Instance.new("Fire")
	fire.Parent = bob
	fire.Size = 10
	
	
	
	
	local thing = game:GetService("Debris")
	thing:AddItem(bob, 5)
	thing:AddItem(chat, 5)
	if player.leaderstats.Exp.Value >= 500 then
	player.leaderstats.Exp.Value = player.leaderstats.Exp.Value - 500
	else
		local game = game:GetService("Chat")
		game:Chat(player.Character.Head, "You dont have enough")
		game.Players.LocalPlayer.Backpack.LocalScript.Disabled = true
	
	end
	end

end)

You've misspelled the method.

The correct spelling is IsDescendantOf, ending ant not ent