Can not revoke the granted object privilege

 Database version : Oracle 10g Release : 10.2.0.1.0
Operating System : Linux Centos 4.3

Dear all,

I need to grant an object privilege to the role I have created. So I issued this statement through SQLPlus (using SYSTEM user):

SQL> GRANT REFERENCES ON merchandise.master_city TO merchandise_role
  2  /

Grant succeeded.

And it said "Grant succeeded".
But, when I tried to revoke it later (using SYSTEM user):

SQL> REVOKE REFERENCES ON merchandise.master_city FROM merchandise_role
  2  /
REVOKE REFERENCES ON merchandise.master_city FROM merchandise_role
*
ERROR at line 1:
ORA-01927: cannot REVOKE privileges you did not grant

And I got an error. It said that I have not granted the privilege (Certainly, I have).