Keep an entity's angles opposite to a player's mouse aim vector?

So, I want to keep an entity permanently rotated opposite to the player’s aim vector ( Adquired with Player.GetAimVector ), however this

[lua]

local plyaim = ply:GetAimVector():Angle() --Gets aim vector, turns it into an angle
self.Entity:SetAngles( plyaim + Angle( 180, 180, 180 ) )

[/lua]

However this just makes the entity roll uncontrollably. This is, by the way, put into the entity’s Think function.

Any ideas?

May I ask what it is for? Did you try it without adding an angle? Also you might want to try Angle.RotateAroundAxis

Just do…

[lua]( ply:GetAimVector( ) * -1 ):Angle( )[/lua]

If you want to negate the angle.