From the original Struts ticket - https://issues.apache.org/struts/browse/WW-1495:

When the action class extends a class that uses generics (e.g. public class ManageEmployeeAction extends CrudAction<Employee, Long> { }) and data is sent from the web page (using Struts Forms built by the tag library) to be set in a property from the class that is inherited from the superclass and defined as generic (e.g., protected I[] ids;), OGNL cannot determine the type of the property, falls back to String[] and throws a ClassCastException.

This issue has been further discussed in the forum: http://forums.opensymphony.com/thread.jspa?threadID=48752. Attachments are provided to illustrate the issue and propose a starting point for the fix. However, it should NOT be considered a patch, since the creator of this "hack" doesn't have enough knowledge of OGNL and Generics to assure its general correctness.