write_index error "don't know how to serialize this type of index"

Here is my code:
```
index = faiss.index_factory(d, "OPQ16_512,IVF1024,PQ16")
co = faiss.GpuClonerOptions()
co.useFloat16 = False
co.usePrecomputed = False
co.indicesOptions = faiss.INDICES_CPU
index = faiss.index_cpu_to_gpu(res, 0, index, co)

index.train(xt)
del xt

index.add(xb)

faiss.write_index(index, 'faiss.index')  # error happens here
```