diff --git a/python/ray/rllib/optimizers/async_samples_optimizer.py b/python/ray/rllib/optimizers/async_samples_optimizer.py
index d245f48de..039fea346 100644
--- a/python/ray/rllib/optimizers/async_samples_optimizer.py
+++ b/python/ray/rllib/optimizers/async_samples_optimizer.py
@@ -84,7 +84,8 @@ class AsyncSamplesOptimizer(PolicyOptimizer):
                                          learner_queue_size)
         self.learner.start()
 
-        assert len(self.remote_evaluators) > 0
+        if len(self.remote_evaluators) == 0:
+            logger.warning("Config num_workers=0 means training will hang!")
 
         # Stats
         self._optimizer_step_timer = TimerStat()