[Zope] SQL Method cache settings

[-]

hi all
i have a SQL Method that's cached.  however, error occurred intermittently. 
when i disable the cache setting in the advance tab, everything works
perfectly.  anybody know why?

setup
zope-2.1.6, postgresql.

pointers/docs, etc welcome

thanks

the traceback follows

<P>
  <STRONG>Error Type: KeyError</STRONG><BR>
  <STRONG>Error Value: ('select *\012from msg\012where \012obj_id = 2790', 100)</STRONG><BR> 
  </P>
 


<!--
Traceback (innermost last):
  File /usr/local/Zope/lib/python/ZPublisher/Publish.py, line 214, in publish_module
  File /usr/local/Zope/lib/python/ZPublisher/Publish.py, line 179, in publish
  File /usr/local/Zope/lib/python/Zope/__init__.py, line 202, in zpublisher_exception_hook
    (Object: ElementWithAttributes)
  File /usr/local/Zope/lib/python/ZPublisher/Publish.py, line 165, in publish
  File /usr/local/Zope/lib/python/ZPublisher/mapply.py, line 160, in mapply
    (Object: view_msg_html)
  File /usr/local/Zope/lib/python/ZPublisher/Publish.py, line 102, in call_object
    (Object: view_msg_html)
  File /usr/local/Zope/lib/python/OFS/DTMLDocument.py, line 166, in __call__
    (Object: view_msg_html)
  File /usr/local/Zope/lib/python/DocumentTemplate/DT_String.py, line 502, in __call__
    (Object: view_msg_html)
  File /usr/local/Zope/lib/python/DocumentTemplate/DT_In.py, line 633, in renderwob
    (Object: qry_msg_by_id(obj_id=obj_id))
  File /usr/local/Zope/lib/python/DocumentTemplate/DT_Util.py, line 335, in eval
    (Object: qry_msg_by_id(obj_id=obj_id))
    (Info: qry_msg_by_id)
  File <string>, line 0, in ?
  File /usr/local/Zope/lib/python/Shared/DC/ZRDB/DA.py, line 429, in __call__
    (Object: qry_msg_by_id)
  File /usr/local/Zope/lib/python/Shared/DC/ZRDB/DA.py, line 362, in _cached_result
    (Object: qry_msg_by_id)
KeyError: (see above)
 -- 
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