GraphArtist fails

The following code fails with the error below.

import matplotlib as mpl
mpl.use('cairo')
import graph_tool.all as gt
import matplotlib.pyplot as plt

import numpy as np
import cairo

width = 640
height = 480
fig = plt.figure()

g = gt.collection.data["celegansneural"]
state = gt.minimize_nested_blockmodel_dl(g, deg_corr=True)

renderer = mpl.backends.backend_cairo.RendererCairo(fig.dpi)
renderer.set_width_height(width, height)
data = np.zeros((width, height, 4), dtype=np.uint8)

surface = cairo.ImageSurface.create_for_data(
    data, cairo.FORMAT_ARGB32, width, height)
renderer.set_ctx_from_surface(surface)

gt.draw_hierarchy(state, mplfig=fig)

fig.draw(renderer)

Traceback (most recent call last):
  File "graph-tool-bug.py", line 26, in <module>
    fig.draw(renderer)
  File "/Users/rafa/Library/Python/3.6/lib/python/site-packages/matplotlib/artist.py", line 55, in draw_wrapper
    return draw(artist, renderer, *args, **kwargs)
  File "/Users/rafa/Library/Python/3.6/lib/python/site-packages/matplotlib/figure.py", line 1475, in draw
    renderer, self, artists, self.suppressComposite)
  File "/Users/rafa/Library/Python/3.6/lib/python/site-packages/matplotlib/image.py", line 141, in _draw_list_compositing_images
    a.draw(renderer)
  File "/Users/rafa/Library/Python/3.6/lib/python/site-packages/matplotlib/artist.py", line 55, in draw_wrapper
    return draw(artist, renderer, *args, **kwargs)
  File "/Users/rafa/Library/Python/3.6/lib/python/site-packages/matplotlib/axes/_base.py", line 2607, in draw
    mimage._draw_list_compositing_images(renderer, self, artists)
  File "/Users/rafa/Library/Python/3.6/lib/python/site-packages/matplotlib/image.py", line 141, in _draw_list_compositing_images
    a.draw(renderer)
  File "/usr/local/lib/python3.6/site-packages/graph_tool/draw/cairo_draw.py", line 1570, in draw
    self.vorder, self.eorder, self.nodesfirst, self.kwargs)
  File "/usr/local/lib/python3.6/site-packages/graph_tool/draw/cairo_draw.py", line 690, in cairo_draw
    vattrs, vdefaults = _attrs(vprops, "v", g, vcmap)
  File "/usr/local/lib/python3.6/site-packages/graph_tool/draw/cairo_draw.py", line 484, in _attrs
    nattrs[int(attr)] = _prop(d, g, _convert(attr, v, cmap))
  File "/usr/local/lib/python3.6/site-packages/graph_tool/draw/cairo_draw.py", line 323, in _convert
    cmap, alpha = cmap
ValueError: too many values to unpack (expected 2)

I was able to determine that the error is caused by a call in cairo_draw.py, line 1564:

    cairo_draw(self.g, pos, ctx, self.vprops, self.eprops,
               self.vorder, self.eorder, self.nodesfirst, self.kwargs)

Changing it to the following solves the problem:

    cairo_draw(self.g, pos, ctx, self.vprops, self.eprops,
               self.vorder, self.eorder, self.nodesfirst, **self.kwargs)