That probably means the clojure block is returning nil as the value. I don't know what it means to return something in clojure, but here is what you have to do with Python, for example.
#+BEGIN_SRC python :results value code
[1, 2, 3]
#+END_SRC
#+RESULTS:
#+begin_src python
None
#+end_src
You have to explicitly return a value to see it.
#+BEGIN_SRC python :results value code
return [1, 2, 3]
#+END_SRC
#+RESULTS:
#+begin_src python
[1, 2, 3]
#+end_src
John
-----------------------------------
Professor John Kitchin (he/him/his)
Doherty Hall A207F
Department of Chemical Engineering
Carnegie Mellon University
Pittsburgh, PA 15213
412-268-7803