What does this "attempt to concatenate field '?' (a nil value)" error message mean?

So I'm trying to get my gun system to create an impact sound and play a random sound from a table of sound IDs. It works.. ish. Sometimes the sound will play, but when they don't it throws this error. I'm detecting impacts with the client and firing it over using a RemoteEvent to the server to execute the hit code. Got no idea what's going on here.

--// Client Code

hitEvent:FireServer(p,cross,angle,normal,"Part",h)

--// Server Code

local Meta = Instance.new("Sound")
Meta.Name = "Crack" 

if h.Material == Enum.Material.Grass or h.Material == Enum.Material.Ice or h.Material == Enum.Material.Fabric or h.Material == Enum.Material.Pebble then
    Meta = Crack:FindFirstChild('Crack') or Instance.new('Sound',Crack)
    Meta.Name = 'Crack'
    Meta.SoundId = "rbxassetid://" .. DirtSounds[math.random(0,#DirtSounds)]
end

if h.Material == Enum.Material.Sand then
    Meta = Crack:FindFirstChild('Crack') or Instance.new('Sound',Crack)
    Meta.Name = 'Crack'
    Meta.SoundId = "rbxassetid://" .. SandSounds[math.random(0,#SandSounds)]
end

if h.Material == Enum.Material.Glass or h.Material == Enum.Material.Neon then
    Meta = Crack:FindFirstChild('Crack') or Instance.new('Sound',Crack)
    Meta.Name = 'Crack'
    Meta.SoundId = "rbxassetid://" .. GlassSounds[math.random(0,#GlassSounds)]
end

if h.Material == Enum.Material.Wood or h.Material == Enum.Material.WoodPlanks then
    Meta = Crack:FindFirstChild('Crack') or Instance.new('Sound',Crack)
    Meta.Name = 'Crack'
    Meta.SoundId = "rbxassetid://" .. WoodSounds[math.random(0,#WoodSounds)]
end

if h.Material == Enum.Material.Metal or h.Material == Enum.Material.CorrodedMetal or h.Material == Enum.Material.DiamondPlate then
    Meta = Crack:FindFirstChild('Crack') or Instance.new('Sound',Crack)
    Meta.Name = 'Crack'
    Meta.SoundId = "rbxassetid://" .. MetalSounds[math.random(0,#MetalSounds)]
end

if h.Material == Enum.Material.Water then
    Meta = Crack:FindFirstChild('Crack') or Instance.new('Sound',Crack)
    Meta.Name = 'Crack'
    Meta.SoundId = "rbxassetid://" .. WaterSounds[math.random(0,#WaterSounds)]
end

if h.Material == Enum.Material.Concrete or h.Material == Enum.Material.Plastic or h.Material == Enum.Material.SmoothPlastic or h.Material == Enum.Material.Slate or h.Material == Enum.Material.Foil or h.Material == Enum.Material.Brick or h.Material == Enum.Material.Granite or h.Material == Enum.Material.Marble or h.Material == Enum.Material.Cobblestone then
    Meta = Crack:FindFirstChild('Crack') or Instance.new('Sound',Crack)
    Meta.Name = 'Crack'
    Meta.SoundId = "rbxassetid://" ..                       ConcreteSounds[math.random(0,#ConcreteSounds)]
end 

    Meta.Volume = math.random(0.8,1)
    Meta.EmitterSize = 10
    Meta.MaxDistance = 30
    Meta.Pitch = math.random(0,0.6)
    Meta.Parent = Crack
    Meta:play()

--// Error Message

17:10:12.344 - Stack Begin
17:10:12.344 - Script 'Players.StoneFox_Alfa.Backpack.Editing Model.Program_Server', Line 442
17:10:12.345 - Stack End
17:10:12.443 - Players.StoneFox_Alfa.Backpack.Editing Model.Program_Server:442: attempt to concatenate field '?' (a nil value)