[Zope] Session & InvalidObjectReference

[p.t.]

I'm trying to remember what was the last SQL query called registering to a 
session the "pointer" to a SQL instance, something like:

anSQLquery = container.DB.SQL.anSQLqury

Howevere, I get this error:

Error Type: InvalidObjectReference
Error Value: Attempt to store an object from a foreign database connection

just when attempting to write to a session the variable anSQLquery.

The traceback diagnostic is not very helping to me; it says:

(Info: (('Products.Transience.TransientObject', 'TransientObject')

referring to:

Connection.py, line 345, in commit

I did examine the code, but unfortunately my limited knowledge of such a 
sophisticated Python code gives me no possibility to uderstand what's wrong.

Any suggestion on how to avoid this error message?
TIA,
p.t.