giving a entity a trail

ive tried using other trail codes from madcows weapons pack and others but mabye its not compatible with it or something every time its says null entity can you please just add a line to the code to make a trail with the colors 217 213 235 255 laser.vmt
trail start size .95 end size .5

heres the code

AddCSLuaFile( “cl_init.lua” )
AddCSLuaFile( “shared.lua” )
include( “shared.lua” )

function ENT:Initialize()

self.Entity:SetModel( "models/weapons/bt_762.mdl" )

self.Entity:PhysicsInit( SOLID_VPHYSICS )
self.Entity:SetSolid( SOLID_NONE )
self.Entity:SetMoveType( MOVETYPE_VPHYSICS )

local phys = self.Entity:GetPhysicsObject( )

if phys:IsValid( ) then
	phys:EnableGravity( false )
	phys:Wake( )
end

self.Start = CurTime( )

end

function ENT:PhysicsUpdate( PhysObj )

if self.Start < CurTime( ) - ( 10 / GetConVarNumber( "phys_timescale" ) ) then
	self.Entity:Remove( )
	return
end

local Trace = {
	start = self.Entity:GetPos( ),
	endpos = self.Entity:GetPos( ) + ( self.Entity:GetForward( ) * 20 ),
	filter = self.Entity
}

local TraceRes = util.TraceLine( Trace )

if TraceRes.Hit or TraceRes.HitWorld then

	if TraceRes.Entity:GetClass( ) ~= "physical_bullet" then

		local Bullet = {
			Src = self.Entity:GetPos( ),
			Dir = self.Entity:GetForward( ),
			Spread = 0,
			Num = 1,
			Damage = self.Damage,
			Force = self.Force,
			Tracer = 0,
			TracerName = "Pistol"
		}
	
		self.Owner:FireBulletsOld( Bullet )

		self.Entity:Remove( )
		
		return false
	
	end
	
end

PhysObj:SetVelocity( self.Entity:GetForward( ) * 10000 )

end