When I run this code and try to access all of the NPCs I get this error code. anything?

Workspace.Alien Rush.Collision:9: attempt to index field '?' (a nil value)

local PhysicsService = game:GetService("PhysicsService")
count = 1

PhysicsService:CreateCollisionGroup("NPCs")
local aliens = script.Parent.Aliens:GetChildren()

while count ~= 14 do
 wait()
 PhysicsService:CollisionGroupContainsPart("NPCs",aliens[count].HumanoidRootPart)
 count = count + 1
end


PhysicsService:CreateCollisionGroup("Obstacles")
local obstacles = script.Parent.Plants:GetChildren()
count = 1

while count ~= 14 do
 wait()
 PhysicsService:CollisionGroupContainsPart("Obstacles",obstacles[count].Part)
 count = count + 1
end

PhysicsService:CollisionGroupSetCollidable("NPCs","Obstacles",false)