pdict.diff

diff --git a/lib/task/sup.ex b/lib/task/sup.ex
index 3070aa2..4cc4968 100644
--- a/lib/task/sup.ex
+++ b/lib/task/sup.ex
@@ -66,10 +66,14 @@ defmodule Task.Sup do
   """
   @spec async(Supervisor.supervisor, module, atom, [term]) :: Task.t
   def async(supervisor, module, fun, args) do
-    { :ok, pid } = Supervisor.start_child(supervisor, [self(), { module, fun, args }])
+    lock = make_ref()
+    Process.put(__MODULE__, lock)
+    { :ok, pid } = Supervisor.start_child(supervisor, [self(), { module, fun, args }, lock])
     ref = Process.monitor(pid)
     send pid, { self(), ref }
     %Task{pid: pid, ref: ref}
+  after
+    Process.delete(__MODULE__)
   end
 
   @doc """
diff --git a/lib/task/supervised.ex b/lib/task/supervised.ex
index 78df117..341f3c7 100644
--- a/lib/task/supervised.ex
+++ b/lib/task/supervised.ex
@@ -5,8 +5,8 @@ defmodule Task.Supervised do
     :proc_lib.start_link(__MODULE__, :noreply, [fun])
   end
 
-  def start_link(caller, fun) do
-    :proc_lib.start_link(__MODULE__, :reply, [caller, fun])
+  def start_link(caller, fun, lock) do
+    :proc_lib.start_link(__MODULE__, :reply, [caller, fun, lock])
   end
 
   def async(caller, { module, fun, args }) do
@@ -27,7 +27,12 @@ defmodule Task.Supervised do
     end
   end
 
-  def reply(caller, mfa) do
+  def reply(caller, mfa, lock) do
+    { :dictionary, dict } = Process.info(caller, :dictionary)
+    if dict[Task.Sup] != lock do
+      exit(:nocaller)
+    end
+
     :erlang.link(caller)
     :proc_lib.init_ack({ :ok, self() })
     async(caller, mfa)