Low down the speed little by little?

Hi! I'm trying to low down the speed of a ParticleEmitter little by little with this script

while true do
	script.Parent.Speed = script.Parent.Speed - NumberRange.new(1,1)
	wait(0.1)
end

On the output it says: " Workspace.Top gun.Nozzle.ParticleEmitter.Script:2: attempt to perform arithmetic on field 'Speed' (a userdata value)" Any solution?

I've never really used particle emitters before, but this code works.

local Val = 50
script.Parent.Speed = NumberRange.new(Val)
while true do
    Val = Val-1
script.Parent.Speed = NumberRange.new(Val)
wait(0.1)
end