TypeError: can't compare datetime.datetime to instancemethod

[ale-rt]

I found this error

2018-04-13 09:23:04 ERROR Zope.SiteErrorLog 1523604184.020.56817560071 http://localhost:8080/Plone/pinnwand/listing_view
Traceback (innermost last):
  Module ZPublisher.Publish, line 138, in publish
  Module ZPublisher.mapply, line 77, in mapply
  Module ZPublisher.Publish, line 48, in call_object
  Module lmu.contenttypes.pinnwand.browser.views, line 107, in __call__
  Module Products.Five.browser.pagetemplatefile, line 125, in __call__
  Module Products.Five.browser.pagetemplatefile, line 59, in __call__
  Module zope.pagetemplate.pagetemplate, line 132, in pt_render
  Module zope.pagetemplate.pagetemplate, line 240, in __call__
  Module zope.tal.talinterpreter, line 271, in __call__
  Module zope.tal.talinterpreter, line 343, in interpret
  Module zope.tal.talinterpreter, line 888, in do_useMacro
  Module zope.tal.talinterpreter, line 343, in interpret
  Module zope.tal.talinterpreter, line 533, in do_optTag_tal
  Module zope.tal.talinterpreter, line 518, in do_optTag
  Module zope.tal.talinterpreter, line 513, in no_tag
  Module zope.tal.talinterpreter, line 343, in interpret
  Module zope.tal.talinterpreter, line 852, in do_condition
  Module zope.tal.talinterpreter, line 343, in interpret
  Module zope.tal.talinterpreter, line 946, in do_defineSlot
  Module zope.tal.talinterpreter, line 343, in interpret
  Module zope.tal.talinterpreter, line 533, in do_optTag_tal
  Module zope.tal.talinterpreter, line 518, in do_optTag
  Module zope.tal.talinterpreter, line 513, in no_tag
  Module zope.tal.talinterpreter, line 343, in interpret
  Module zope.tal.talinterpreter, line 533, in do_optTag_tal
  Module zope.tal.talinterpreter, line 518, in do_optTag
  Module zope.tal.talinterpreter, line 513, in no_tag
  Module zope.tal.talinterpreter, line 343, in interpret
  Module zope.tal.talinterpreter, line 852, in do_condition
  Module zope.tal.talinterpreter, line 343, in interpret
  Module zope.tal.talinterpreter, line 533, in do_optTag_tal
  Module zope.tal.talinterpreter, line 518, in do_optTag
  Module zope.tal.talinterpreter, line 513, in no_tag
  Module zope.tal.talinterpreter, line 343, in interpret
  Module zope.tal.talinterpreter, line 821, in do_loop_tal
  Module zope.tal.talinterpreter, line 343, in interpret
  Module zope.tal.talinterpreter, line 533, in do_optTag_tal
  Module zope.tal.talinterpreter, line 518, in do_optTag
  Module zope.tal.talinterpreter, line 513, in no_tag
  Module zope.tal.talinterpreter, line 343, in interpret
  Module zope.tal.talinterpreter, line 583, in do_setLocal_tal
  Module zope.tales.tales, line 696, in evaluate
   - URL: /home/ale/Code/plone/projects/lmu-iuk.buildout/src/lmu.contenttypes.pinnwand/src/lmu/contenttypes/pinnwand/browser/templates/listing_view.pt
   - Line 82, Column 10
   - Expression: <PythonExpr ( item.isExpired())>
   - Names:
      {'args': (),
       'container': <PinnwandFolder at /Plone/pinnwand>,
       'context': <PinnwandFolder at /Plone/pinnwand>,
       'default': <object object at 0x7f7dbc468560>,
       'here': <PinnwandFolder at /Plone/pinnwand>,
       'loop': {},
       'nothing': None,
       'options': {},
       'repeat': <Products.PageTemplates.Expressions.SafeMapping object at 0x7f7da229b4c8>,
       'request': <HTTPRequest, URL=http://localhost:8080/Plone/pinnwand/listing_view>,
       'root': <Application at >,
       'template': <Products.Five.browser.pagetemplatefile.ViewPageTemplateFile object at 0x7f7da8f80a50>,
       'traverse_subpath': [],
       'user': <PropertiedUser 'admin'>,
       'view': <Products.Five.metaclass.ListingView object at 0x7f7da8273d50>,
       'views': <Products.Five.browser.pagetemplatefile.ViewMapper object at 0x7f7da82c9910>}
  Module zope.tales.pythonexpr, line 59, in __call__
   - __traceback_info__: ( item.isExpired())
  Module <string>, line 1, in <module>
  Module lmu.contenttypes.pinnwand.content, line 44, in isExpired
TypeError: can't compare datetime.datetime to instancemethod

Maybe this is not a bug in the code, but it might be that this is caused by my testing DB which is broken/outdated.
I am just putting it here to note it.