SQL & PL/SQL

Dear all,

what is the err in below code as i have created directory
create directory aak as 'c:\aak'

and created folder on c drive by name aak.

I do not understand where I am wrong as the below code is throwing err as below

ERROR at line 1:
ORA-29280: invalid directory path
ORA-06512: at "SYS.UTL_FILE", line 33
ORA-06512: at "SYS.UTL_FILE", line 436
ORA-06512: at line 4


declare
v_handle utl_file.file_type;
begin
v_handle:=utl_file.fopen('c:\aak','a.txt','W');
utl_file.put_line(v_handle,'line one some text....');
utl_file.put_line(v_handle,'line two some text.....');
utl_file.fclose(v_handle);
end;
/

thanks in advance