Specification:+index asserts - (from_node, to_node) not in self.edges

OOPS-2021EE81 Obtained this oops after adding a blueprint as a dependency.

AssertionError:

    Traceback (most recent call last):
  Module zope.publisher.publish, line 134, in publish
    result = publication.callObject(request, obj)
  Module canonical.launchpad.webapp.publication, line 498, in callObject
    return mapply(ob, request.getPositionalArguments(), request)
  Module zope.publisher.publish, line 109, in mapply
    return debug_call(obj, args)
   - __traceback_info__: <security proxied zope.browserpage.metaconfigure.SimpleViewClass from /srv/launchpad.net/production/launchpad-rev-13405.1/lib/lp/blueprints/browser/../templates/specification-index.pt instance at 0x10d1b6d0>
  Module zope.publisher.publish, line 115, in debug_call
    return obj(*args)
  Module canonical.launchpad.webapp.publisher, line 308, in __call__
    return self.render()
  Module canonical.launchpad.webapp.publisher, line 293, in render
    return self.template()
  Module zope.app.pagetemplate.viewpagetemplatefile, line 83, in __call__
    return self.im_func(im_self, *args, **kw)
  Module zope.app.pagetemplate.viewpagetemplatefile, line 51, in __call__
    sourceAnnotations=getattr(debug_flags, 'sourceAnnotations', 0),
  Module zope.pagetemplate.pagetemplate, line 113, in pt_render
    strictinsert=0, sourceAnnotations=sourceAnnotations)()
  Module zope.tal.talinterpreter, line 271, in __call__
    self.interpret(self.program)
  Module zope.tal.talinterpreter, line 343, in interpret
    handlers[opcode](self, args)
  Module zope.tal.talinterpreter, line 888, in do_useMacro
    self.interpret(macro)
  Module zope.tal.talinterpreter, line 343, in interpret
    handlers[opcode](self, args)
  Module zope.tal.talinterpreter, line 533, in do_optTag_tal
    self.do_optTag(stuff)
  Module zope.tal.talinterpreter, line 518, in do_optTag
    return self.no_tag(start, program)
  Module zope.tal.talinterpreter, line 513, in no_tag
    self.interpret(program)
  Module zope.tal.talinterpreter, line 343, in interpret
    handlers[opcode](self, args)
  Module zope.tal.talinterpreter, line 533, in do_optTag_tal
    self.do_optTag(stuff)
  Module zope.tal.talinterpreter, line 522, in do_optTag
    self.interpret(program)
  Module zope.tal.talinterpreter, line 343, in interpret
    handlers[opcode](self, args)
  Module zope.tal.talinterpreter, line 946, in do_defineSlot
    self.interpret(slot)
  Module zope.tal.talinterpreter, line 343, in interpret
    handlers[opcode](self, args)
  Module zope.tal.talinterpreter, line 852, in do_condition
    self.interpret(block)
  Module zope.tal.talinterpreter, line 343, in interpret
    handlers[opcode](self, args)
  Module zope.tal.talinterpreter, line 533, in do_optTag_tal
    self.do_optTag(stuff)
  Module zope.tal.talinterpreter, line 518, in do_optTag
    return self.no_tag(start, program)
  Module zope.tal.talinterpreter, line 513, in no_tag
    self.interpret(program)
  Module zope.tal.talinterpreter, line 343, in interpret
    handlers[opcode](self, args)
  Module zope.tal.talinterpreter, line 531, in do_optTag_tal
    self.no_tag(stuff[-2], stuff[-1])
  Module zope.tal.talinterpreter, line 513, in no_tag
    self.interpret(program)
  Module zope.tal.talinterpreter, line 343, in interpret
    handlers[opcode](self, args)
  Module zope.tal.talinterpreter, line 742, in do_insertStructure_tal
    structure = self.engine.evaluateStructure(expr)
  Module zope.tales.tales, line 696, in evaluate
    return expression(self)
   - /srv/launchpad.net/production/launchpad-rev-13405.1/lib/lp/blueprints/browser/../templates/specification-index.pt
   - Line 289, Column 8
   - Expression: <PathExpr standard:u'context/@@+deptreeimgtag'>
   - Names:
      {'args': (),
       'context': <Specification 38137 u'topic-oneiric-community-web-projects' for u'ubuntu'>,
       'default': <object object at 0x2aead5f39220>,
       'loop': {},
       'nothing': None,
       'options': {},
       'repeat': {},
       'request': <lp.blueprints.publisher.BlueprintsBrowserRequest instance URL=https://blueprints.launchpad.net/ubuntu/+spec/topic-oneiric-community-web-projects/+index>,
       'template': <z3c.ptcompat.ViewPageTemplateFile object at 0xa421e50>,
       'view': <zope.browserpage.metaconfigure.SimpleViewClass from /srv/launchpad.net/production/launchpad-rev-13405.1/lib/lp/blueprints/browser/../templates/specification-index.pt object at 0x10d1b6d0>,
       'views': <zope.app.pagetemplate.viewpagetemplatefile.ViewMapper object at 0x120c5350>}
  Module zope.tales.expressions, line 217, in __call__
    return self._eval(econtext)
  Module zope.tales.expressions, line 211, in _eval
    return ob()
  Module canonical.launchpad.webapp.publisher, line 308, in __call__
    return self.render()
  Module lp.blueprints.browser.specification, line 1307, in render
    image_map = self.renderGraphvizGraph('cmapx').decode('UTF-8')
  Module lp.blueprints.browser.specification, line 1248, in renderGraphvizGraph
    input = self.getDotFileText().encode('UTF-8')
  Module lp.blueprints.browser.specification, line 1238, in getDotFileText
    specgraph = self.makeSpecGraph()
  Module lp.blueprints.browser.specification, line 1232, in makeSpecGraph
    graph.addDependencyNodes(self.context)
  Module lp.blueprints.browser.specification, line 995, in addDependencyNodes
    self.walkSpecsMakingNodes(spec, get_related_specs_fn, link_nodes_fn)
  Module lp.blueprints.browser.specification, line 1024, in walkSpecsMakingNodes
    link_nodes_fn(node, self.newOrExistingNode(related_spec))
  Module lp.blueprints.browser.specification, line 994, in link_nodes_fn
    self.link(dependency, node)
  Module lp.blueprints.browser.specification, line 984, in link
    assert (from_node, to_node) not in self.edges