ValueError: cannot map over XCom with custom key 'b' from <Task(_PythonDecoratedOperator): t>`

import pendulum
from airflow.decorators import dag, task, task_group


@dag(schedule=None, start_date=pendulum.datetime(2022, 1, 1))
def pipeline():
    @task
    def t():
        return {"b": [2, 3]}

    @task_group()
    def tg(a, b):
        pass

    tg.partial(a=1).expand(b=t()["b"])

pipeline()

Traceback (most recent call last):
  File "/opt/airflow/task-sdk/src/airflow/sdk/definitions/mappedoperator.py", line 132, in ensure_xcomarg_return_value
    ensure_xcomarg_return_value(v)
  File "/opt/airflow/task-sdk/src/airflow/sdk/definitions/mappedoperator.py", line 127, in ensure_xcomarg_return_value
    raise ValueError(f"cannot map over XCom with custom key {key!r} from {operator}")
ValueError: cannot map over XCom with custom key 'b' from <Task(_PythonDecoratedOperator): t>