Custom indexer not working

I have a custom contenttype 'FHNWEvent' with some fields and the IEventBasic-behavior.
Now I would like to override the indexer for field start but it's not working. As you see below i have print statement for debugging but it doesn't get reached. Does someone has an idea why?

There are override for different fields too (e.g. sortable_title) for the same contenttype and they work perfectly.

Interesting code-parts:
profiles/FHNWEvent.xml

        ...
    	<property name="behaviors" purge="false">
    		...
    		<element value="plone.app.event.dx.behaviors.IEventBasic"/>
    		...
    	</property>
    	...

indexer.py

         ...
	@indexer(IFHNWEvent)
	def start_fhnwevent(object, **kw):
		print '----- Debugging'
		start = IEventAccessor(object).start
		
		# some logic

		return start
	...

configure.zcml

	...
	<adapter name="start" factory=".indexer.start_fhnwevent" />
	...

content.py

	...
	@implementer(IFHNWEvent, IEvent)
	class FHNWEvent(Container):
		""" Event """
	...

interfaces.py

	...
	class IFHNWEvent(Interface):
		""" Interface for Event"""

I suspect it is because the IEventBasic behavior does not define any factory like for instance IRichTextBehavior does, which adapts to IDexterityContent.

Also the default indexer for start it's defined for IDXEvent from which IEventBasic derives:

class IEventBasic(model.Schema, IDXEvent)

So you could probably change you content.py like that:

	...
	@implementer(IFHNWEvent, IDXEvent)
	class FHNWEvent(Container):
		""" Event """
	...

Or either the interfaces.py:

	...
	class IFHNWEvent(IDXEvent):
		""" Interface for Event"""

Thank you for your suggestions! Using IDXEvent instead of Interface as parent to IFHNWEvent worked for me.

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