roc() crashes when called without a selection

>>> roc(out, p_signal, p_bkg, 'pred_is_vbf', axes=ax)

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-59-17f6f9ace8c6> in <module>
      3 import matplotlib.pyplot as plt
      4 fig, ax = plt.subplots()
----> 5 roc(out, p_signal, p_bkg, 'pred_is_vbf', axes=ax)

~/repositories/httnn/lib/python3.5/site-packages/nnfwtbn/plot.py in roc(df, signal_process, background_process, discriminant, steps, selection, min, max, axes, weight)
    352         max = discriminant(df).max()
    353 
--> 354     df = df[selection(df)]
    355 
    356     signal_effs = []

~/repositories/httnn/lib/python3.5/site-packages/nnfwtbn/cut.py in __call__(self, dataframe)
     77             return pd.Series(np.ones(len(dataframe), dtype='bool'))
     78 
---> 79         return self.func(dataframe)
     80 
     81     def select(self, dataframe):

~/repositories/httnn/lib/python3.5/site-packages/nnfwtbn/plot.py in <lambda>(d)
    340     # Handle selection
    341     if selection is None:
--> 342         selection = Cut(lambda d: variable(d) * 0 == 0)
    343     elif not isinstance(selection, Cut):
    344         selection = Cut(selection)

NameError: name 'variable' is not defined