linearmodels.PanelOLS with KeyError: 'This `Formula` instance does not have structure @ `1`.'

from statsmodels.datasets import grunfeld
data = grunfeld.load_pandas().data
data = data.set_index(['firm','year'])

from linearmodels import PanelOLS
mod = PanelOLS.from_formula('invest ~ value + capital + EntityEffects + TimeEffects', 
                           data=data) 

and have:

KeyError: 'This Formula instance does not have structure @ 1.'

full output: full output