import awkward as ak
import numpy as np
form = ak.forms.UnionForm(
    "i8",
    "i64",
    [
        ak.forms.IndexedOptionForm('i64', ak.forms.EmptyForm(), form_key="nones"),
        ak.forms.UnmaskedForm(ak.forms.NumpyForm("int64", form_key="ints")),
        ak.forms.UnmaskedForm(ak.forms.ListOffsetForm(
            "i64",
            ak.forms.NumpyForm("float64", form_key="floats"),
            form_key="offsets")),
    ],
    form_key="union"
)
        
        
virtual_buffers = {
    "nones-index": lambda: np.array([-1], dtype=np.int64),
    "ints-data": lambda: np.array([0, 1], dtype=np.int64),
    "floats-data": lambda: np.array([1., 2.], dtype=np.float64),
    "offsets-offsets": lambda: np.array([0,2], dtype=np.int64),
    "union-tags": lambda: np.array([1, 2], dtype=np.int8),
    "union-index": lambda: np.array([0, 0], dtype=np.int64),
}

eager_buffers = {k: v() for k,v in virtual_buffers.items()}

# This works
array = ak.from_buffers(form, 2, eager_buffers)
print(array)

# This doesn't
array = ak.from_buffers(form, 2, virtual_buffers)
print(array)