Missing rows from an Oracle table during Sqoop import

Create an Oracle table such as:
create table table1 (col1 number, col2 number);

Insert some rows such as:
insert into table1 values (null, null);
insert into table1 values (1, 1);
insert into table1 values (null, null);
insert into table1 values (2, 2);
commit;

Perform a Sqoop import of this table with only 1 mapper:
sqoop import ... --table table1 --num-mappers=1 --split-by col1

You'll get a single output file containing 4 rows; one for each of the rows you inserted.
(This is what you'd expect.)

However, now perform the same export, but allow 4 mappers.
One of the files will contain the row "1,1" one of the files will contain the row "2,2" and the other two files will be zero bytes long.
I'd expect there to be two files containing "null,null".