@jone 

```
 Module zope.tales.tales, line 696, in evaluate
   - URL: /Users/maethu/pkgs/ftw.book/ftw/book/browser/keywords.pt
   - Line 25, Column 24
   - Expression: <PathExpr standard:u'view/keywords'>
   - Names:
      {'args': (),
       'container': <Book at /Plone/example-book>,
       'context': <Book at /Plone/example-book>,
       'default': <object object at 0x102bb6bb0>,
       'here': <Book at /Plone/example-book>,
       'loop': {},
       'nothing': None,
       'options': {},
       'repeat': <Products.PageTemplates.Expressions.SafeMapping object at 0x1083cf368>,
       'request': <HTTPRequest, URL=http://localhost:8080/Plone/example-book/tabbed_view/listing>,
       'root': <Application at >,
       'template': <Products.Five.browser.pagetemplatefile.ViewPageTemplateFile object at 0x1063bf210>,
       'traverse_subpath': [],
       'user': <PropertiedUser 'admin'>,
       'view': <Products.Five.metaclass.KeywordsTab object at 0x1086452d0>,
       'views': <Products.Five.browser.pagetemplatefile.ViewMapper object at 0x1087e5310>}
  Module zope.tales.expressions, line 217, in __call__
  Module Products.PageTemplates.Expressions, line 155, in _eval
  Module Products.PageTemplates.Expressions, line 117, in render
  Module ftw.book.browser.keywords, line 30, in keywords
TypeError: 'Missing.Value' object is not iterable
```

This happends on a upgraded book. 
There's no value on the brain... I guess you need to reindex (INCL. METADATA) the `book_keywords` index in a upgrade step. So there's is a proper default value, like empty list/tuple, or add a condition (I'll prefer the first solution). 
After a catalog_rebuild it worked in my case