Welds not working?

I'm not very good at welds, as I do not know how to use them, but I pulled this directly off the wiki, and it won't work:

            local function weldBetween(a,b)
            local weld = Instance.new("ManualWeld")
            weld.Part0 = a
            weld.Part1 = b
            weld.C0 = CFrame.new()
            weld.C1 = b.CFrame:inverse() * a.CFrame
            weld.Parent = a
            return weld;
            end
            weldBetween(throwingHandle, hit.Parent)

I get the error: bad cast