got EXP-00091 warning

Greetings,

When I export schema containing some tables, I got EXP-00091 warnings. The log is saying:
"
..
JServer Release 9.2.0.1.0 - Production
Export done in US7ASCII character set and AL16UTF16 NCHAR character set server uses WE8ISO8859P1 character set (possible charset conversion)
. exporting pre-schema procedural objects and actions
. exporting foreign function library names for user XXX
. exporting PUBLIC type synonyms
. exporting private type synonyms
. exporting object type definitions for user XXX
About to export XXX's objects ...
. exporting database links
. exporting sequence numbers
. exporting cluster definitions
. about to export XXX's tables via Conventional Path ...
. . exporting table CHEMISTRY 0 rows exported
EXP-00091: Exporting questionable statistics.
. . exporting table CHEMISTRY_ALIAS 0 rows exported
EXP-00091: Exporting questionable statistics.
. . exporting table DESTINATION_PLATE_WELL 0 rows exported
EXP-00091: Exporting questionable statistics. ...
"

I check oracle document and it says:
"
EXP-00091 Exporting questionable statistics

    Cause: Export was able to export statistics, but the statistics may not be useable. The statistics are questionable because one or more of the following happened during export: a row error occurred, client character set or NCHARSET does not match with the server, a query clause was specified on export, only certain partitions or subpartitions were exported, or a fatal error occurred while processing a table.

    Action: To export non-questionable statistics, change the client character set or NCHARSET to match the server, export with no query clause, or export complete tables. If desired, import parameters can be supplied so that only non-questionable statistics will be imported, and all questionable statistics will be recalculated.
"

So I think the problem is chararter set not matching. So how can I change client character set?

Thanks,
Jin

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