Value error when iterating over vertex properties

Something simple like:

g = gt.Graph()
v1 = g.add_vertex()
v2 = g.add_vertex()
label = g.new_vertex_property("string")
for name in label:
    pass

results in an in "ValueError: Invalid vertex index: 2"

Looks just like a one off error.

gt.__version__ gives '2.13 (commit c15ecea6, Tue Mar 1 19:10:53 2016 +0100)'

I may try to look into the source and see if I can fix the issue and push it if I have time. The traceback is below.

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-17-b49b77d7d85a> in <module>()
----> 1 for _ in label:
      2     pass
      3 

/usr/lib/python2.7/dist-packages/graph_tool/__init__.pyc in __getitem__(self, k)
    524         except ArgumentError:
    525             try:
--> 526                 k = self.__key_convert(k)
    527                 return self.__map[k]
    528             except ArgumentError:

/usr/lib/python2.7/dist-packages/graph_tool/__init__.pyc in __key_convert(self, k)
    502             except:
    503                 raise ArgumentError
--> 504             key = self.__g().vertex(key)
    505         return key
    506 

/usr/lib/python2.7/dist-packages/graph_tool/__init__.pyc in vertex(self, i, use_index, add_missing)
   1779                 self.add_vertex(int(i) - self.num_vertices(use_index) + 1)
   1780                 return self.vertex(int(i), use_index)
-> 1781             raise ValueError("Invalid vertex index: %d" % int(i))
   1782         return v
   1783 

ValueError: Invalid vertex index: 2