Enabling constraint runs out of space..??

[Horace]

I have a table in a dedicated tablespace

SELECT TABLE_NAME,TABLESPACE_NAME FROM USER_TABLES WHERE TABLE_NAME = 'N_CONTACT_HISTORY';

TABLE_NAME                     TABLESPACE_NAME
------------------------------ ------------------------------
N_CONTACT_HISTORY              MART

The table has a primary key constraint on it

SELECT CONSTRAINT_NAME FROM USER_CONSTRAINTS WHERE TABLE_NAME = 'N_CONTACT_HISTORY';

CONSTRAINT_NAME
------------------------------

PK_N_CONTACT_HISTORY

The table owner has a different tablespace as default

SELECT USERNAME,DEFAULT_TABLESPACE FROM DBA_USERS; (output edited)

USERNAME                       DEFAULT_TABLESPACE
------------------------------ ------------------------------

N_OWNER                        USERS

Here's the problem.........

ALTER TABLE N_CONTACT_HISTORY ENABLE VALIDATE CONSTRAINT PK_C_CONTACT_HISTORY;

ORA-01652: unable to extend temp segment by 64 in tablespace USERS

If the table is in tablespace MART, why is the constraint enablement using USERS??

I think I'll go RTFM whilst someone comes up with a good answer. Thanks.