ProjectMilestone:+global-actions: oops rendering the structural subscription link

OOPS-1952DX458
Traceback (most recent call last):
...
lib/lp/registry/browser/../templates/milestone-index.pt
Line 331, Column 6
Expression: <PathExpr standard:u'view/milestone/@@+global-actions'>
   - Names:
      {'args': (),
       'context': <lp.registry.model.milestone.ProjectMilestone object at 0x12a7fcd0>,
...
Module lp.bugs.browser.structuralsubscription, line 391, in subscribe_to_bug_mail
return Link('#', text, icon='add', hidden=True, enabled=self._enabled)
Module lp.bugs.browser.structuralsubscription, line 384, in _enabled
pillar = IStructuralSubscriptionTargetHelper(sst).pillar
Module zope.site.hooks, line 95, in adapter_hook
return siteinfo.adapter_hook(interface, object, name, default)
Module zope.component.security, line 79, in factory
ob = original_factory(*args)
Module lp.bugs.model.structuralsubscription, line 259, in __init__
self.join = (StructuralSubscription.milestone == target)
Module storm.references, line 204, in __eq__
return self._relation.get_where_for_local(other)
Module storm.references, line 537, in get_where_for_local
return compare_columns(self.local_key, remote_variables)
Module storm.expr, line 1451, in compare_columns
value = columns[0].variable_factory(value=value)
Module storm.variables, line 342, in parse_set
% (type(value), value))
TypeError: Expected int, found <type 'zope.security._proxy._Proxy'>: <lp.registry.model.milestone.ProjectMilestone object at 0x12a7fcd0>