Natural join query returns wrong result

0: jdbc:drill:schema=dfs> select * from `t1.json`;
+------------+------------+------------+
|     a1     |     b1     |     c1     |
+------------+------------+------------+
| 1          | 1          | 2015-01-01 |
| 2          | 2          | 2015-01-02 |
+------------+------------+------------+
2 rows selected (0.087 seconds)

0: jdbc:drill:schema=dfs> select * from `t2.json`;
+------------+------------+------------+
|     a1     |     b1     |     c1     |
+------------+------------+------------+
| 1          | 1          | 2015-01-01 |
+------------+------------+------------+
1 row selected (0.112 seconds)

0: jdbc:drill:schema=dfs> select * from `t1.json` natural join `t2.json`;
+------------+------------+------------+------------+------------+------------+
|     a1     |     b1     |     c1     |    a10     |    b10     |    c10     |
+------------+------------+------------+------------+------------+------------+
+------------+------------+------------+------------+------------+------------+
No rows selected (0.223 seconds)

0: jdbc:drill:schema=dfs> select * from `t1.json` t1, `t2.json` t2 where t1.a1=t2.a1 and t1.b1=t2.b1 and t1.c1=t2.c1;
+------------+------------+------------+------------+------------+------------+
|     a1     |     b1     |     c1     |    a10     |    b10     |    c10     |
+------------+------------+------------+------------+------------+------------+
| 1          | 1          | 2015-01-01 | 1          | 1          | 2015-01-01 |
+------------+------------+------------+------------+------------+------------+
1 row selected (0.732 seconds)