IndexOutOfBoundsException

I am using quaere to get the common elements from 2 ArrayLists that hold objects of the same type.
I get out of bounds exception that does not make any logical sense.
The first ArrayList has 20 elements and the second 9 and i get the following error:

java.lang.IndexOutOfBoundsException: Index: 16, Size: 16

If it would be of any help I am using the following query:

from("concept").in(s.getConcepts()).
join("supportedconcept").in(listSupportedConcepts) .on("concept.id").
equals("supportedconcept.id").select("concept").it erator()

I would really appreciate your help.