SQL Developer Data Modeler

The oddest thing. I just used SQL Developer Datamodeler (4.0.2.840) to generate some DDL. This is the result (anonimized):

CREATE TABLE abcd.efgh
  (
    ID NUMBER NOT NULL ,
    aa1 NVARCHAR2 (2) ,
    aa2 NVARCHAR2 (8) ,
    aa3 NVARCHAR2 ,
    aa4 NVARCHAR2 (2) ,
    aa5 NVARCHAR2 (2) ,
    aa6 NVARCHAR2 ,
    aa7 NVARCHAR2 ,
    aa8 NVARCHAR2 ,
    aa9 NVARCHAR2 (3) ,
    aa0 NUMBER (15,7) ,
    SDDateIns DATE ,
    SDDateUpd DATE
  ) ;

As you can see, some of the NVARCHAR2 fields have a length specified, some don't. However, this is reverse engineerd from an MS-SQL table that has fields with VARCHAR datatypes that ALL have length specified. It seems that whenever the original length = 1, the generated DDL has no length specified. All other lengths are nicely specified. So I expected this result:

CREATE TABLE abcd.efgh
  (
    ID NUMBER NOT NULL ,
    aa1 NVARCHAR2 (2) ,
    aa2 NVARCHAR2 (8) ,
    aa3 NVARCHAR2 (1) ,
    aa4 NVARCHAR2 (2) ,
    aa5 NVARCHAR2 (2) ,
    aa6 NVARCHAR2 (1) ,
    aa7 NVARCHAR2 (1) ,
    aa8 NVARCHAR2 (1) ,
    aa9 NVARCHAR2 (3) ,
    aa0 NUMBER (15,7) ,
    SDDateIns DATE ,
    SDDateUpd DATE
  ) ;

Any ideas about what I'm doing wrong?