ORA-01790 error on union all with nullable bind variables

Hello everyone!

Does anybody know why this query fails with ORA-01790?

```
>>> import cx_Oracle
>>> con = cx_Oracle.connect('django/[email protected]/orcl')
>>> cur = con.cursor()
>>> cur.execute('SELECT 1 FROM dual UNION ALL SELECT :arg0 FROM dual',
arg0=None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
cx_Oracle.DatabaseError: ORA-01790: expression must have same datatype
as corresponding expression
```

It seems to me that bind variables passes as empty string '' and not as
NULL value.
Moreover, when I execute this query in oracle sqldeveloper I had the
same exception. Is it some oracle "feature"?

P.S.: The purpose of this question is the bug in django:
https://code.djangoproject.com/ticket/22669


-- 
Mikhail <[email protected]>



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