left-join on char column fails about tech.ml.dataset HOT 1 CLOSED

(j/left-join :a ds1 ds1)
;; => left-outer-join [9 2]:
;;    | :a | :right.a |
;;    |----|----------|
;;    |  1 |        1 |
;;    |  2 |        2 |
;;    |  3 |        3 |
;;    |  4 |        4 |
;;    |  5 |        5 |
;;    |  6 |        6 |
;;    |  7 |        7 |
;;    |  8 |        8 |
;;    |  9 |        9 |


(j/left-join :a ds2 ds2)
;; => left-outer-join [9 2]:
;;    | :a | :right.a |
;;    |----|----------|
;;    |  0 |        0 |
;;    |  9 |        9 |
;;    |  8 |        8 |
;;    |  7 |        7 |
;;    |  6 |        6 |
;;    |  5 |        5 |
;;    |  4 |        4 |
;;    |  3 |        3 |
;;    |  2 |        2 |