vertex_color in draw_hierarchy not working

When passing a vertex property as vertext_color parameter to function draw_hierarchy, it spills the following error:

.../graph_tool/draw/cairo_draw.pyc in draw_hierarchy(state, pos, layout, beta, ealpha, halpha, subsample_edges, deg_order, deg_size, vsize_scale, hsize_scale, empty_branches, verbose, **kwargs)
   1723             vertex_color[v] = vfcolor[v]
   1724         elif vcolor is not None:
-> 1725             vertex_color[v] = vfcolor
   1726         else:
   1727             vertex_color[v] = vcmap(b[v] / (B - 1) if B > 1 else 0)

.../graph_tool/__init__.pyc in __setitem__(self, k, v)
    443             self.__map[key] = v
    444         except TypeError:
--> 445             self.__map[key] = _convert(self, v)
    446 
    447     def __repr__(self):

.../graph_tool/__init__.pyc in _convert(prop, val)
    242     vtype = _python_type(prop.value_type())
    243     if type(vtype) is tuple:
--> 244         return [vtype[1](x) for x in val]
    245     if vtype is object:
    246         return val

TypeError: 'NoneType' object is not iterable

It does it for string and int vertex property types.