ODP.NET

Hi,

i have a problem with a select statement
we use odp.net Version 2.102.2.20
our database server version is
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - 64bi
PL/SQL Release 10.2.0.1.0 - Production
CORE 10.2.0.1.0 Production
TNS for Solaris: Version 10.2.0.1.0 - Production
NLSRTL Version 10.2.0.1.0 - Production

The problem is:
when i do a first select like this
begin open :Liste for select distinct(MA_ROLLE) from ap.benutzerrollen where MA_ROLLE like :ROLLE order by MA_ROLLE desc;end;
the variable :MA_ROLLE is a varchar(50) INPUT and the variable :Liste is a RefCursor
the value of the parameter is 'GA'
when after that i do a second select but the parameter value is longer than the previous one for example 'Tester' i receive the following error:
ORA-01460: unimplemented or unreasonable conversion requested.

Is it a known problem, is there a workaround?

thank you