diff --git a/src/lein_monolith/task/each.clj b/src/lein_monolith/task/each.clj
index d61b32c..86c6671 100644
--- a/src/lein_monolith/task/each.clj
+++ b/src/lein_monolith/task/each.clj
@@ -165,8 +165,12 @@
   (when-not *task-file-output*
     (throw (IllegalStateException.
              (str "Cannot run task without bound *task-file-output*: " (pr-str cmd)))))
-  (let [env (@#'eval/overridden-env eval/*env*)
-        ^Process proc (.exec (Runtime/getRuntime) (into-array String cmd) env (io/file eval/*dir*))]
+  (let [cmd (into-array String cmd)
+        env (into-array String (@#'eval/overridden-env eval/*env*))
+        proc (.exec (Runtime/getRuntime)
+                    ^{:tag "[Ljava.lang.String;"} cmd
+                    ^{:tag "[Ljava.lang.String;"} env
+                    (io/file eval/*dir*))]
     (.addShutdownHook (Runtime/getRuntime)
                       (Thread. (fn [] (.destroy proc))))
     (with-open [out (.getInputStream proc)
@@ -187,6 +191,13 @@
           exit-value)))))
 
 
+(def ^:private init-lock
+  "An object to lock on to ensure that projects are not initialized
+  concurrently. This prevents the mysterious 'unbound fn' errors that sometimes
+  crop up during parallel execution."
+  (Object.))
+
+
 (defn- apply-subproject-task
   "Applies the task to the given subproject."
   [monolith subproject task]
@@ -198,7 +209,8 @@
           (fn inject-profile [p k v] (assoc-in p [:profiles k] v))
           subproject inherited)
         (config/debug-profile "init-subproject"
-          (project/init-project subproject (cons :default (keys inherited))))
+          (locking init-lock
+            (project/init-project subproject (cons :default (keys inherited)))))
         (config/debug-profile "apply-task"
           (binding [eval/*dir* (:root subproject)]
             (lein/resolve-and-apply subproject task)))))))