Failing joins on field that is nullable in one table but not in the other

[j-chmielewski]

Given following tables (postgresql db):

example> \d table1
+----------+---------+-------------+
| Column   | Type    | Modifiers   |
|----------+---------+-------------|
| id       | integer | not null    |
| field    | integer |             |
+----------+---------+-------------+
Indexes:
    "table1_pkey" PRIMARY KEY, btree (id)

example> \d table2
+----------+---------+-------------+
| Column   | Type    | Modifiers   |
|----------+---------+-------------|
| id       | integer | not null    |
| field    | integer | not null    |
+----------+---------+-------------+
Indexes:
    "table2_pkey" PRIMARY KEY, btree (id)

Following code fails:

join(table1, table2, 'field')

With this error message:

TypeError                                 Traceback (most recent call last)
<ipython-input-14-2a191a07eec9> in <module>()
----> 1 join(database.table1, database.table2, 'field')

/home/user/anaconda/envs/env/lib/python2.7/site-packages/blaze/expr/collections.pyc in join(lhs, rhs, on_left, on_right, how, suffixes)
    435                          (lhs, rhs))
    436     if types_of_fields(on_left, lhs) != types_of_fields(on_right, rhs):
--> 437         raise TypeError("Schema's of joining columns do not match")
    438     _on_left = tuple(on_left) if isinstance(on_left, list) else on_left
    439     _on_right = (tuple(on_right) if isinstance(on_right, list)

TypeError: Schema's of joining columns do not match

Types returned by types_of_fields function:

types_of_fields('field', database.table1) #  ?int32
types_of_fields('field', database.table2) #  ctype("int32")