I think I found why the target unitary needs to be a `TwoQubitWeylDecomposition` class as the `.a`, `.b`, and `.c` attributes allow for the access of the second `np.mod((d[:3]+d[3])/2, 2*np.pi)`. This argument is needed for the `TwoQubitBasisDecomposer()`, and so the target unitary needs to be a `TwoQubitWeylDecomposition`. I hope this helps. :)