facet_grid fail with a category type

[mxardre]

`
File "", line 102, in
print(p)

File "python3.7/site-packages/ggplot/ggplot.py", line 116, in repr
self.make()

File "python3.7/site-packages/ggplot/ggplot.py", line 636, in make
layer.plot(ax, facetgroup, self._aes, **kwargs)

File "python3.7/site-packages/ggplot/geoms/geom_histogram.py", line 48, in plot
params['bins'] = np.arange(np.min(x), np.max(x) + self.params['binwidth'], self.params['binwidth'])

ValueError: arange: cannot compute length`

I am trying to do a facet_grid of a dataframe. To order the facet I have follow this : https://stackoverflow.com/questions/42763667/ordering-the-facet-in-python-ggplot
This throw the error above and the facet fail.
Does the pb come from the category type of the pd.dataframe?