[ins] In [1]: import awkward as ak
         ...: ak.__version__
Out[1]: '2.1.1'

[ins] In [3]: empty = ak.Array([])
         ...: ak.flatten(empty[empty])
---------------------------------------------------------------------------
AxisError                                 Traceback (most recent call last)
Cell In[3], line 1
----> 1 ak.flatten(empty[empty])

File ~/Programs/anaconda3/envs/tree2/lib/python3.11/site-packages/awkward/operations/ak_flatten.py:164, in flatten(array, axis, highlevel, behavior)
     12 """
     13 Args:
     14     array: Array-like data (anything #ak.to_layout recognizes).
   (...)
    158      999]
    159 """
    160 with ak._errors.OperationErrorContext(
    161     "ak.flatten",
    162     {"array": array, "axis": axis, "highlevel": highlevel, "behavior": behavior},
    163 ):
--> 164     return _impl(array, axis, highlevel, behavior)

File ~/Programs/anaconda3/envs/tree2/lib/python3.11/site-packages/awkward/operations/ak_flatten.py:232, in _impl(array, axis, highlevel, behavior)
    229     return wrap_layout(out, behavior, highlevel, like=array)
    231 else:
--> 232     out = ak._do.flatten(layout, axis)
    233     return wrap_layout(out, behavior, highlevel, like=array)

File ~/Programs/anaconda3/envs/tree2/lib/python3.11/site-packages/awkward/_do.py:253, in flatten(layout, axis)
    252 def flatten(layout: Content, axis: int = 1) -> Content:
--> 253     offsets, flattened = layout._offsets_and_flattened(axis, 1)
    254     return flattened

File ~/Programs/anaconda3/envs/tree2/lib/python3.11/site-packages/awkward/contents/numpyarray.py:415, in NumpyArray._offsets_and_flattened(self, axis, depth)
    412     return self.to_RegularArray()._offsets_and_flattened(axis, depth)
    414 else:
--> 415     raise ak._errors.wrap_error(
    416         np.AxisError(f"axis={axis} exceeds the depth of this array ({depth})")
    417     )

AxisError: while calling

    ak.flatten(
        array = <Array [] type='0 * int64'>
        axis = 1
        highlevel = True
        behavior = None
    )

Error details: axis=1 exceeds the depth of this array (1)

[ins] In [1]: import awkward as ak
         ...: print(ak.__version__)
1.8.0

[nav] In [2]: empty = ak.Array([])
         ...: ak.flatten(empty[empty])
Out[2]: <Array [] type='0 * unknown'>