decrease column size

Hi,

     I tried decrease column size from varchar2(3000) to varchar2(1000), but got error: Failed to commit: ORA-01441: cannot decrease column length because some value is too big.

    

     But maxdatalength is 997: select max(length(kuvaus)) as max_length from paikkatietotuote;

                                            MAX_LENGTH

                                                  997

   So there is no data over 997 - why error when decrease column? And how to resolve problem?

Regards Jari