diff --git a/ddpo_pytorch/stat_tracking.py b/ddpo_pytorch/stat_tracking.py
index 4199ab9..ee50034 100644
--- a/ddpo_pytorch/stat_tracking.py
+++ b/ddpo_pytorch/stat_tracking.py
@@ -9,6 +9,8 @@ class PerPromptStatTracker:
         self.stats = {}
 
     def update(self, prompts, rewards):
+        prompts = np.array(prompts)
+        rewards = np.array(rewards)
         unique = np.unique(prompts)
         advantages = np.empty_like(rewards)
         for prompt in unique: