PL/SQL :: Oracle 12c - Generated By Default As Identity Generates Duplicates?

I am trying the new Oracle 12c and its feature to create columns with the keyword IDENTITY. I create a table CREATE TABLE xt (a NUMBER GENERATED by default AS IDENTITY PRIMARY KEY, b VARCHAR2(10)); 

And populate it, sometimes specifying the value for a and sometimes relying on the system to generate the value: 

INSERT INTO xt (b) values ('a');INSERT INTO xt (b, a) values ('b', default);INSERT INTO xt (b, a) values ('c', 3);INSERT INTO xt (b) values ('d');INSERT INTO xt (b) values ('e'); 

The problem is that the fourth INSERT fails because the system tries to use the value 3 which is already taken.  The fifth statement gets the value 4.

And the table now contains: A B - -1 a 2 b 3 c 4 e Is there something I am missing? I understood that by specifying BY DEFAULT, I would be allowed to sometimes specify values on my own without them interfering with the generated values. I now that the same thing works correctly in MySQL (where I would get five rows from 1 to 5 with the same INSERT statements) /nikos