subtask-help-for expects a map, so let's give it one.

src/leiningen/help.clj

@@ -30,10 +30,11 @@
       (formatted-docstring command docstring padding))))
 
 (defn- get-subtasks-and-docstrings-for [task]
+  (into {}
         (map (fn [subtask]
                (let [m (meta subtask)]
                  [(str (:name m)) (first (.split (:doc m "") "\n"))]))
-       (:subtasks (meta task))))
+             (:subtasks (meta task)))))
 
 (defn subtask-help-for
   [task-ns task]