diff --git a/baselines/common/vec_env/subproc_vec_env.py b/baselines/common/vec_env/subproc_vec_env.py
index 83192cb..a593695 100644
--- a/baselines/common/vec_env/subproc_vec_env.py
+++ b/baselines/common/vec_env/subproc_vec_env.py
@@ -37,6 +37,7 @@ class SubprocVecEnv(VecEnv):
         envs: list of gym environments to run in subprocesses
         """
         self.waiting = False
+        self.closed = False
         nenvs = len(env_fns)
         self.remotes, self.work_remotes = zip(*[Pipe() for _ in range(nenvs)])
         self.ps = [Process(target=worker, args=(work_remote, remote, CloudpickleWrapper(env_fn)))
@@ -53,22 +54,26 @@ class SubprocVecEnv(VecEnv):
         VecEnv.__init__(self, len(env_fns), observation_space, action_space)
 
     def step_async(self, actions):
+        self._assert_not_closed()
         for remote, action in zip(self.remotes, actions):
             remote.send(('step', action))
         self.waiting = True
 
     def step_wait(self):
+        self._assert_not_closed()
         results = [remote.recv() for remote in self.remotes]
         self.waiting = False
         obs, rews, dones, infos = zip(*results)
         return np.stack(obs), np.stack(rews), np.stack(dones), infos
 
     def reset(self):
+        self._assert_not_closed()
         for remote in self.remotes:
             remote.send(('reset', None))
         return np.stack([remote.recv() for remote in self.remotes])
 
     def close_extras(self):
+        self.closed = True
         if self.waiting:
             for remote in self.remotes:
                 remote.recv()
@@ -78,7 +83,11 @@ class SubprocVecEnv(VecEnv):
             p.join()
 
     def get_images(self):
+        self._assert_not_closed()
         for pipe in self.remotes:
             pipe.send(('render', None))
         imgs = [pipe.recv() for pipe in self.remotes]
         return imgs
+
+    def _assert_not_closed(self):
+        assert not self.closed, "Trying to operate on a SubprocVecEnv after calling close()"