Clojurians Log v2

I'm trying the incanter.core/$map function in Incanter 1.9.0 on the example code in the docstring and get a NullPointerException:

ns> (use '(incanter core datasets))
nil
ns> (def cars (get-dataset :cars))
#'ns/cars
ns> (i/$map (fn [s] (/ s)) :speed cars)
NullPointerException   clojure.lang.Numbers.ops (Numbers.java:1013)

Anyone has any idea of what I'm doing wrong?