Problem in propagateDeleteForObject: 'Array is empty' ?

Hello,

I have a complex model, modelling a tree structure with persistence in the
database. In the tree, it can be the case that a node has more than one parent.
Assume the following configuration:

N1.children = N2,N3,N4
N2.children = N4,N5,N6

=> N4.parents = N1,N2

When adding one child to more than one parents, then deleting one of the
parent nodes, I get problems. Which means, when deleting N1 or N2, I get
the following exception:

1) testValueNodeHierarchies(com.tecc.avida.model.test.ValueNodeTests)java.lang.IllegalArgumentException: Array is empty
	at com.webobjects.foundation.NSArray.objectAtIndex(NSArray.java:510)
	at com.webobjects.eocontrol._EOCheapCopyMutableArray.objectAtIndex(_EOCheapCopyMutableArray.java:98)
	at com.webobjects.eocontrol.EOClassDescription.propagateDeleteForObject(EOClassDescription.java:516)
	at com.webobjects.eocontrol.EOCustomObject.propagateDeleteWithEditingContext(EOCustomObject.java:688)
	at com.webobjects.eocontrol.EOClassDescription.propagateDeleteForObject(EOClassDescription.java:523)
	at com.webobjects.eocontrol.EOCustomObject.propagateDeleteWithEditingContext(EOCustomObject.java:688)
	at com.webobjects.eocontrol.EOEditingContext.propagateDeletesUsingTable(EOEditingContext.java:2231)
	at com.webobjects.eocontrol.EOEditingContext._processDeletedObjects(EOEditingContext.java:2193)
	at com.webobjects.eocontrol.EOEditingContext._processRecentChanges(EOEditingContext.java:1744)
	at com.webobjects.eocontrol.EOEditingContext._prepareForPushChanges(EOEditingContext.java:3257)
	at com.webobjects.eocontrol.EOEditingContext.saveChanges(EOEditingContext.java:3213)
	...

The children/parent relationships between nodes are in fact flattened 
relationships, which run via a many-to-many table. Did someone happen 
to run into this problem before?

Best,
J.L.Simon