[SOLVED] Why does my game/studio crash when attempting to run this script?

I think the issue is terrain:ReplaceMaterial

I have rewritten it several times, but running the script will always end up crashing the game. I am attempting to get a region of terrain replaced with a different material when clicking the button

local open = true

--GFX VARIABLES--
local Bloom = game.Lighting.Bloom
local Blur = game.Lighting.Blur
local SunRays = game.Lighting.SunRays
local Shadows = game.Lighting.GlobalShadows
--GFX VARIABLES--


--GRASS POSITION SETTINGS--
local min = game.Workspace.Part1
local max = game.Workspace.Part2

local Region = Region3.new(min.Position, max.Position)
local materialToReplace = Enum.Material.Grass
local replacementMaterial = Enum.Material.LeafyGrass
--GRASS POSITION SETTINGS--


script.Parent.MouseButton1Click:Connect(function()
    if open == true then
        Bloom.Enabled = false
        Blur.Enabled = false
        SunRays.Enabled = false
        Shadows = false
        Region = Region:ExpandToGrid(4)
        workspace.Terrain:ReplaceMaterial(Region, 4, materialToReplace, replacementMaterial)
        open = false
        script.Parent.Text = "Fast GFX [Active]"
    else
        Bloom.Enabled = true
        Blur.Enabled = true
        SunRays.Enabled = true
        Shadows = true
        Region = Region:ExpandToGrid(4)
        workspace.Terrain:ReplaceMaterial(Region, 4, materialToReplace, replacementMaterial)
        open = true
        script.Parent.Text = "Fast GFX [InActive]"
    end
end)