"TypeError" on "find_vertex" call

from graph_tool.all import *
G = Graph(directed=True)
vprop = G.new_vertex_property("int")                
for i in range(0,20):
    v = G.add_vertex()
    vprop[v] = i
print find_vertex(G, vprop, 2)

Traceback (most recent call last):
  File "models.py", line 121, in <module>
    get_road_network()
  File "models.py", line 100, in get_road_network
    print find_vertex(G, vprop, 2)
  File "/home/aluuu/workspace/work/receiver/.env/lib/python2.7/site-packages/graph_tool/util/__init__.py", line 58, in find_vertex
    (val, val))
TypeError: No registered converter was able to extract a C++ reference to type graph_tool::GraphInterface from this Python object of type Graph