geom_bar and stat_summary and faceting causes ignored ValueError warnings

[TyberiusPrime]

The following minimal example

import pandas as pd
import plotnine
import patsy
df = pd.DataFrame(patsy.demo_data("a",'b','x','y', 'z'))
p = plotnine.ggplot(df)
p += plotnine.geom_bar(plotnine.aes('a','y'), stat='stat_summary')
p += plotnine.facet_wrap('b')
p.save("test.png")

causes repeated

ValueError                                Traceback (most recent call last)
ValueError: Buffer dtype mismatch, expected 'Python object' but got 'long'

Exception ignored in: 'pandas._libs.lib.is_bool_array'
ValueError: Buffer dtype mismatch, expected 'Python object' but got 'long'

messages (one per facet/bar combination?)
But does produce the requested file.

Seems to occur in line 221 of stat_summary.py:
merged = summary.merge(unique, on=['group', 'x'])

[TyberiusPrime]

Apperantly it's a pandas bug that disappears when upgrading to 0.23.4 from 0.22
Possibly pandas-dev/pandas#17187