my script runs properly when going through it the first time but the second time i get an error?

16:11:26.077 - The Parent property of Model1 is locked, current parent: NULL, new parent Workspace

model1 = game.ReplicatedStorage.Model1:Clone()
model2 = game.ReplicatedStorage.Model2:Clone()

repeat
    model1.Parent = workspace
    model1:MakeJoints()
    wait ("10")
    if model1 then
        model1:destroy()
    end
    wait ("10")
    model2.Parent = workspace
    model2:MakeJoints()
    wait ("10")
    if model2 then
        model2:destroy()
    end
    wait ("10")

until false