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