We have a synonym : TESTSYNONYM in OLTP database (on NT)

This is connect to another database <on Unix> using link <DBLINK>

One develeoper is trying to insert data into it form OLTP

insert into TESTSYNONYM values ('B940614BEC9D7464E0340003BA219335' , ....) - works

But not the below

insert into TESTSYNONYM values (sys_guid() , ....) - doesn't work
ERROR at line 1:
ORA-00984: column not allowed here
ORA-02063: preceding line from <DBLINK>

Any help is appreciated