jax-ml/jax · error · ValueError

Cannot concatenate an empty list of vectors

Error message

Cannot concatenate an empty list of vectors

What it means

vector_concat builds a result by recursively concatenating, starting from vectors[0]; an empty sequence has no element type or length to anchor on, so it raises rather than returning an undefined value.

Source

Thrown at jax/experimental/mosaic/gpu/utils.py:2126

  v_ty = ir.VectorType(v.type)
  if len(v_ty.shape) != 1:
    raise NotImplementedError(f"Only 1D vectors are supported {v_ty}")
  [v_len] = v_ty.shape
  slice_length = len(range(v_len)[s])
  return vector.extract_strided_slice(
      ir.VectorType.get((slice_length,), v_ty.element_type),
      v,
      [s.start or 0],
      [slice_length],
      [1],
  )


def vector_concat(
    vectors: Sequence[ir.Value[ir.VectorType]],
) -> ir.Value[ir.VectorType]:
  if not vectors:
    raise ValueError("Cannot concatenate an empty list of vectors")
  vty = vectors[0].type
  if not isinstance(vty, ir.VectorType):
    raise ValueError("Cannot concatenate non-vector values")
  vty = ir.VectorType(vty)
  if vty.rank != 1:
    raise NotImplementedError("Only 1D vectors are supported")
  for v in vectors:
    if v.type.element_type != vty.element_type:
      raise ValueError("Cannot concatenate vectors of different element types")
    if v.type.rank != 1:
      raise ValueError("Can only concatenate 1D vectors")
  return _vector_concat_rec(vectors)


def _vector_concat_rec(
    vectors: Sequence[ir.Value[ir.VectorType]],
) -> ir.Value[ir.VectorType]:
  match vectors:

View on GitHub (pinned to 1e1c6a8fc0)

Solutions

1. Guard the call site: skip concatenation or return an undef/zero vector when the list is empty
2. Fix the loop/range producing the list so it yields at least one fragment
3. Pad the sequence dimension so fragment lists are never empty

Example fix

# before
out = vector_concat(parts)  # parts == []
# after
out = vector_concat(parts) if parts else llvm.mlir_undef(ir.VectorType.get((0,), f32))

assert vectors, 'vector_concat received empty list'
result = vector_concat(vectors)

When it happens

Trigger: Calling vector_concat([]) — usually because a list comprehension over blocks/fragments produced zero elements (empty range, empty tile list, filtered-out everything).

Common situations: Concatenating per-iteration partial vectors inside a loop whose trip count is 0 (e.g. zero-length sequence dimension); dynamic shapes producing empty fragments at runtime; off-by-one slice producing an empty list.

Related errors

• Need at least one array to concatenate
• Cannot concatenate non-vector values
• Cannot concatenate vectors of different element types
• Can only concatenate 1D vectors
• concatenate expects at least one operand, got 0.

AI-assisted analysis of jax-ml/jax@1e1c6a8fc0 (2026-08-27). Data as JSON: /api/errors/4c4b728412c100eb. Report an issue: GitHub.