SQL & PL/SQL

I have a table as defined below (table a).
I want to change the logging of the lob (DATA) from nologging to logging.
I use the alter command:
alter table a modify lob (DATA) store as (nocache logging);
This doesn't work. I've tried numerous variations of this command but i can't get it working.
How do I do.

CREATE TABLE a
(
b NUMBER(10) NOT NULL,
c BLOB NOT NULL
)
TABLESPACE ss
PCTUSED 40
PCTFREE 10
INITRANS 1
MAXTRANS 255
STORAGE (
INITIAL 64K
MINEXTENTS 1
MAXEXTENTS 2147483645
PCTINCREASE 0
FREELISTS 1
FREELIST GROUPS 1
BUFFER_POOL DEFAULT
)
LOGGING
NOCOMPRESS
LOB (DATA) STORE AS
( TABLESPACE BLOBSPACE
ENABLE STORAGE IN ROW
CHUNK 8192
PCTVERSION 0
NOCACHE
NOLOGGING
STORAGE (
INITIAL 800M
MINEXTENTS 1
MAXEXTENTS 2147483645
PCTINCREASE 0
BUFFER_POOL DEFAULT
)
)
CACHE
NOPARALLEL
MONITORING;


/Robert