Undoing Things, fo' Real.

[ikg]

So I've spawned an entity, and I can't undo it without getting this error:

[i]:1: attempt to call method 'OnUndo' (a nil value)[/i]

Hmm. Here's my code, and if anybody could point at me
the newbie mistake I'm making, well...that'd just make my day.

		barrel = ents.Create("prop_physics")
		barrel:SetModel("models/props_c17/oildrum001.mdl")
		barrel:SetPos( ply:GetEyeTrace().HitPos + ply:GetAimVector() )
		barrel:Spawn()


		undo.Create("Barrel")
			undo.AddEntity( barrel )
			undo.SetPlayer( ply )
		undo.Finish()

Thanks to you gentlemen who kindly tolerate newbie questions.