Help me with this error?

The error was in the in game Dev Console so I couldn't copy and paste it, I took a screenshot https://gyazo.com/abef379904022add480adbc87cbe5977 Idk what this error means, help me pls... This is the script that made the error:

function weld()
	local parts,last = {}
	local function scan(parent)
		for _,v in pairs(parent:GetChildren()) do
			if (v:IsA("BasePart")) or (v:IsA("UnionOperation")) then
				if (last) then
					local w = Instance.new("Weld")
					w.Name = ("%s_Weld"):format(v.Name)
					w.Part0,w.Part1 = last,v
					w.C0 = last.CFrame:inverse()
					w.C1 = v.CFrame:inverse()
					w.Parent = last
					print(""..v.Name.." has been welded in "..script.Parent.Name..".")
				end
				last = v
				table.insert(parts,v)
			end
			scan(v)
		end
	end
	scan(script.Parent)
	for _,v in pairs(parts) do
		v.Anchored = false
	end
end

weld()
script:Remove()

It means you set the value of a weld to a nil value, possibly by using a variable that had no value, or a false/existent value.

Post any welding code here