Ich versuche, mithilfe von NetworkX parallele Kanten zwischen zwei Knoten hinzuzufügen, aber der folgende Fehler tritt auf. Was mache ich falsch

import networkx as nx
import graphviz

g1 = nx.MultiGraph()

node1 = 'a'
node2 = 'b'

g1.add_edge(node1,node2,key='one')
g1.add_edge(node1,node2,key='two')

A = nx.to_agraph(g1)
A.add_subgraph()

A.draw('test2.png', prog='dot')

Error

Traceback (most recent call last):
  File "test2.py", line 12, in <module>
    A = nx.to_agraph(g1)
  File "C:\python27\lib\site-packages\networkx-1.11rc1-py2.7.egg\networkx\drawing\nx_agraph.py", line 152, in to_agraph
    A.add_edge(u,v,key=str(key),**str_edgedata)
  File "C:\python27\lib\site-packages\pygraphviz\agraph.py", line 481, in add_edge
    eh = gv.agedge(self.handle, uh, vh, key, _Action.find)
KeyError: 'agedge: no key'