[Python] how to fix `"Series[Any]" not callable [operator]` mypy error

For the code below

import pandas as pd

df_data = pd.DataFrame(
[
{"a": 2, "b": 1},
{"a": 2, "b": 10},
{"a": 3, "b": 77},
]
)


df_data.groupby("a").size().to_frame("size")


mypy complains

toy.py:12: error: "Series[Any]" not callable [operator]
Found 1 error in 1 file (checked 1 source file)


I know the complain is due to to_frame call, not sure why or how to fix?

versions:

• pandas: 2.2.2
• pandas-stub: 2.2.2.240807
• mypy: 1.11.2

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