语句报错，请助

[root@laf ~]# su - db2inst1
[db2inst1@laf ~]$ db2start
07/26/2013 06:57:57     0   0   SQL1063N  DB2START processing was successful.
SQL1063N  DB2START processing was successful.
[db2inst1@laf ~]$ db2 connect to testdb     Database Connection Information
Database server        = DB2/LINUX 9.5.0
SQL authorization ID   = DB2INST1
Local database alias   = TESTDB
[db2inst1@laf ~]$ db2 "select stmt_text from table(mon_get_pkg_cache_stmt(NULL,NULL,NULL,-2)) as T where rows_read <> 0 order by rows_returned/rows_read asc fetch first 10 rows only"
SQL0206N  "NULL" is not valid in the context where it is used.  SQLSTATE=42703
[db2inst1@laf ~]$
当我执行蓝色部分的语句时，老是报红色部分的错误，什么原因导致的呢，向大家求助