graph_draw with subplots

I am trying to plot different graphs using plt.subplots. Following code throws an error:

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

g1 = gt.collection.data['karate']
g2 = gt.collection.data['dolphins']

ax = plt.subplot(1, 2, 1)
gt.graph_draw(g1, mplfig = ax)
ax = plt.subplot(1, 2, 2)
gt.graph_draw(g2, mplfig = ax)
ax = plt.subplot(2, 2, 3)

plt.savefig('test.pdf')

The error is given below:

Traceback (most recent call last):
  File "test.py", line 20, in <module>
    plt.savefig('abc.pdf')
  File "/usr/lib/python3/dist-packages/matplotlib/pyplot.py", line 688, in savefig
    res = fig.savefig(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/matplotlib/figure.py", line 1565, in savefig
    self.canvas.print_figure(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/matplotlib/backend_bases.py", line 2232, in print_figure
    **kwargs)
  File "/usr/lib/python3/dist-packages/matplotlib/backends/backend_cairo.py", line 448, in print_pdf
    return self._save(fobj, 'pdf', *args, **kwargs)
  File "/usr/lib/python3/dist-packages/matplotlib/backends/backend_cairo.py", line 512, in _save
    self.figure.draw (renderer)
  File "/usr/lib/python3/dist-packages/matplotlib/artist.py", line 61, in draw_wrapper
    draw(artist, renderer, *args, **kwargs)
  File "/usr/lib/python3/dist-packages/matplotlib/figure.py", line 1159, in draw
    func(*args)
  File "/usr/lib/python3/dist-packages/matplotlib/artist.py", line 61, in draw_wrapper
    draw(artist, renderer, *args, **kwargs)
  File "/usr/lib/python3/dist-packages/matplotlib/axes/_base.py", line 2324, in draw
    a.draw(renderer)
  File "/usr/lib/python3/dist-packages/graph_tool/draw/cairo_draw.py", line 1570, in draw
    self.vorder, self.eorder, self.nodesfirst, self.kwargs)
  File "/usr/lib/python3/dist-packages/graph_tool/draw/cairo_draw.py", line 692, in cairo_draw
    vdefs = _attrs(_vdefaults, "v", g, vcmap)[1]
  File "/usr/lib/python3/dist-packages/graph_tool/draw/cairo_draw.py", line 486, in _attrs
    defaults[int(attr)] = _convert(attr, v, cmap)
  File "/usr/lib/python3/dist-packages/graph_tool/draw/cairo_draw.py", line 323, in _convert
    cmap, alpha = cmap
ValueError: not enough values to unpack (expected 2, got 0)