a RuntimeError I don't understand

Hello Tiago:
While attempting to save a binary structure I encountered this error
RuntimeError: Pickling of "graph_tool.libgraph_tool_core.Vertex" instances is not enabled (http://www.boost.org/libs/python/doc/v2/pickle.html)

I don't understand why I was able to pickle.dump a stochastic block model (which contains vertices) and not this structure which contains a list of vertices.
The url given in references returns a 404

Thanks in advance

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