diff --git a/hw1/cs285/infrastructure/replay_buffer.py b/hw1/cs285/infrastructure/replay_buffer.py
index 60148e79a..0fce19e79 100644
--- a/hw1/cs285/infrastructure/replay_buffer.py
+++ b/hw1/cs285/infrastructure/replay_buffer.py
@@ -18,7 +18,7 @@ def __init__(self, max_size=1000000):
         self.terminals = None
 
     def __len__(self):
-        if self.obs:
+        if self.obs is not None:
             return self.obs.shape[0]
         else:
             return 0