Problem in running CSALTER script in Oracle

First of all, I run the csscan then I run the CSALTER script. But i got the following error,
Checking or Converting phrase did not finish successfully
No database (national) character set will be altered
CSALTER finished unsuccessfully.
What I had to do is to take is to take a dump for the tables having exceptional data and convertible data. Then TRUNCATE those tables and run the CSALTER script.
SQL> @?/rdbms/admin/csalter.plb