Need help creating a non lag script to make parts far away from the player transparent?

i want to make it so that the player goes through a grass field without much lag, But it doesn't work, as it cannot understand global 'avatar' at line 10, a nil value. What do?

distance = 200

avatar = game.Players:FindFirstChildOfClass("Player")
grass = workspace.GrassField:GetChildren()

for g=1, #grass do
    grasspatch = grass[g]
end

if (avatar.character.Torso.Position - grass.Position).magnitude <= distance then
    grasspatch.transparency = 0
else
    grasspatch.transparency = 1
end