Why lowercase 'i' mapped to '?' after doing NLS_UPPER

Can someone please explain why lowercase i in string abcdefghijklmnopqrstuvwxyz is mapped to ? whereas all other alphabets are correctly mapped to corresponding uppercase alphabets.

SQL> ALTER SESSION SET NLS_SORT="xturkish";

Session altered.

SQL> 
SQL> begin
  2   DBMS_OUTPUT.PUT_LINE('NLS_UPPER(abcdefghijklmnopqrstuvwxyz) = '
  3                || NLS_UPPER('abcdefghijklmnopqrstuvwxyz'));
  4  end;
  5  /
NLS_UPPER(abcdefghijklmnopqrstuvwxyz) = ABCDEFGH?JKLMNOPQRSTUVWXYZ

PL/SQL procedure successfully completed.

SQL> 

Update: If nls_sort is set to BINARY, the mapping happens as expected, lowercase i maps to a plane English alphabet I