[graph-tool] the graph becomes non-existent

When I create a graph like this: self.graph = Graph(directed=True) self.is_on = self.graph.new_vertex_property("boolean") and want to use it in another class method: self.is_on.set_value(False) I get the error: libcore.set_vertex_property(g._Graph__graph, _prop("v", g, self), val) AttributeError: 'NoneType' object has no attribute '_Graph__graph' -- Sent from: http://main-discussion-list-for-the-graph-tool-project.982480.n3.nabble.com/