[Zope] Sequence.sort error

[Zanotti Michele]

Hi all
This dtml instruction:

<dtml-in
expr="_.sequence.sort(myCatalog(),((_.string.split(ordper)[0],'nocase','asc'
),(_.string.split(ordper)[1],'nocase','asc')))" size=num start=query_start>

Gives me (Zope 2.6.3) this error: 

Traceback (innermost last): 
Module ZPublisher.Publish, line 150, in publish_module 
Module ZPublisher.Publish, line 114, in publish 
Module Zope.App.startup, line 202, in zpublisher_exception_hook 
Module ZPublisher.Publish, line 98, in publish 
Module ZPublisher.mapply, line 88, in mapply 
Module ZPublisher.Publish, line 39, in call_object 
Module OFS.DTMLMethod, line 127, in __call__ 
Module DocumentTemplate.DT_String, line 474, in __call__ 
Module DocumentTemplate.DT_In, line 443, in renderwb 
Module DocumentTemplate.DT_Util, line 201, in eval
__traceback_info__: ordper 
Module <string>, line 2, in f 
Module DocumentTemplate.sequence.SortEx, line 103, in sort 
TypeError: unhashable type (Also, an error occurred while attempting to
render the standard error message.) 

In Zope 2.5 this code works (no errors), why?
Thanks
Michele