NullPointerException after simple clojure program -

i working on learning clojure, , have run nullpointerexception seems have nothing code. program runs completion before producing error. code:

; solves collatz conjecture  ; return 1 step in sequence (defn collatz-step [n]    (if (= (rem n 2) 0)     (/ n 2)     (+ 1 (* 3 n))))  ; recurse on numbers (defn collatz [n]    (if (= n 1)     (println "all done!")     ((println (format "n = %d" n))       (collatz (collatz-step n)))))  ; input , run (println "enter positive number:") (collatz (read-string (read-line)))                                                                                   

is there i'm missing?

when line runs:

((println (format "n = %d" n))   (collatz (collatz-step n))) 

the println , colatz finish leving form this:

 (return-value-of-println return-value-of-collatz) 

println returns nil yielding:

 (nil return-value-of-collatz) 

which function call function nil resulting in npe

take out ()

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clojure not have tail call elimination, changing recursive call collatz recur keep blowing stack on large values of n