My CFraming will not work out as planned... Could someone help?

So I am making a door that if you push a button a door will open up. One door will open, but the other one won't. The error I got was " 18:55:09.277 - Workspace.YES.Script:9: attempt to index nil with 'CFrame' "

here's my code:

local door_a = game.Workspace:WaitForChild("DOOR_A")
local door_b = game.Workspace:WaitForChild("DOOR_B")
local click = script.Parent:WaitForChild("ClickDetector")
local example_a = game.Workspace:WaitForChild("EXAMPLE_A")
local example_b game.Workspace:WaitForChild("EXAMPLE_B")
local ts = game:GetService("TweenService")
click.MouseClick:Connect(function(player)
    ts:Create(door_a,TweenInfo.new(.5,Enum.EasingStyle.Sine),{CFrame = example_a.CFrame}):Play()
    ts:Create(door_b,TweenInfo.new(.5,Enum.EasingStyle.Sine),{CFrame = example_b.CFrame}):Play()
end)