ORA-01400: cannot insert NULL into

Why can't I insert a blank string into a NOT NULL colum?

insert into t1 values(2,'');
ORA-01400: cannot insert NULL into ("TEST1"."T1"."C2")

My table is:
create table t1(c1 int,c2 varchar2(10) not null);

Any idea?

for Oracle a blank string is a NULL. So you will not be able to insert a blank into a NOT NULL column

You add a NOT NULL constraint and then inserting a NULL value??
Are you trying to do something like this?
insert into t1 values(2,' ');
This will work as there is a space.