jax-ml/jax · error · ValueError

Indices must not be empty

Error message

Indices must not be empty

What it means

Public wrapper store_scatter rejects an empty indices sequence — scatter with no indices is meaningless and would make the indexed shape ambiguous.

Source

Thrown at jax/_src/pallas/mosaic/sc_primitives.py:396

    ref: Ref,
    indices: Sequence[jax.Array],
    x: jax.Array,
    *,
    mask: jax.Array | None = None,
) -> None:
  """Scatters an array to a ref.

  Args:
    ref: The ref in ``VMEM`` to scatter to.
    indices: A sequence of 1D arrays, one for each dimension of ``ref``. Each
      array specifies an index for that dimension. All arrays must have the same
      size.
    x: The array to store.
    mask: An optional boolean array, which specifies which elements to store. If
      ``None``, all elements are stored.
  """
  if not indices:
    raise ValueError("Indices must not be empty")
  ref, transforms = state_primitives.get_ref_and_transforms(
      ref, None, "store_scatter"
  )
  flat_args, tree = jax.tree.flatten((ref, transforms, indices, x, mask))
  _ = scatter_p.bind(*flat_args, tree=tree, add=False)
  return None


def addupdate_scatter(
    ref: Ref,
    indices: Sequence[jax.Array],
    x: jax.Array,
    *,
    mask: jax.Array | None = None,
) -> None:
  """Scatters an array to a ref, atomically adding to existing values.

  Args:

View on GitHub (pinned to 1e1c6a8fc0)

Solutions

1. Pass at least one index or index array; if you meant 'store everything', use a plain store instead of scatter
2. Guard the call: if not indices: use store or skip

Example fix

// before
sc_primitives.store_scatter(ref, [], x)

// after
if not indices:
  ref[...] = x  # or skip
else:
  sc_primitives.store_scatter(ref, indices, x)

if not indices:
    ref[...] = x  # or skip
else:
    store_scatter(ref, indices, x)

def has_indices(indices) -> bool:
    return len(indices) > 0

When it happens

Trigger: Calling store_scatter(ref, [], x) or passing an indices list/tuple that is empty, e.g. built dynamically and accidentally empty (indices=[] instead of None).

Common situations: Loop-generated index lists that end up empty for a block; confusing mask=None (store all) with indices=[] (store none).

Related errors

• {x.shape=} does not match expected shape {expected_shape}
• val.dtype={x.dtype} != ref.dtype={ref.dtype}
• {mask.shape=} does not match expected shape {expected_shape}
• Scatter only supports VectorSubcoreMesh, got {type(ref_aval.
• Scatter only supports storing to VMEM, got {memory_space}

AI-assisted analysis of jax-ml/jax@1e1c6a8fc0 (2026-08-27). Data as JSON: /api/errors/f5472551261f4333. Report an issue: GitHub.