character string buffer too small

Hi ,

please help in fixing this error,

 INSERT INTO PS_EX_SHEET_AW_H

   SELECT   DISTINCT 'QWLARCH',

                     138,

                     A.PTAFTHREAD_ID,

                     A.PTAFPRCS_ID,

                     A.PTAFDEFN_ID,

                     A.ORIGINATORID,

                     A.PTAFREQUESTOR_ID,

                     A.RECNAME,

                     A.PTAFTHREAD_STATUS,

                     A.PTAFPARENT_THREAD,

                     A.PTAFDTTM_MODIFIED,

                     A.SHEET_ID,

                     A.LINE_NBR,

                     DBMS_LOB.SUBSTR (A.COMMENTS)

     FROM   PS_EX_SHEET_AW A, PS_Q_WL_EX_TMP4 B

    WHERE       A.SHEET_ID = B.SHEET_ID

            AND A.PTAFTHREAD_ID = B.PTAFTHREAD_ID

            AND A.PTAFPRCS_ID = B.PTAFPRCS_ID

            AND A.PTAFDEFN_ID = B.PTAFDEFN_ID

                   *

ERROR at line 15:   DBMS_LOB.SUBSTR (A.COMMENTS)