diff --git a/miles/ray/rollout.py b/miles/ray/rollout.py
index 2a75d492b9..af11accfc3 100644
--- a/miles/ray/rollout.py
+++ b/miles/ray/rollout.py
@@ -1260,10 +1260,18 @@ def _is_zero_std(samples: list[Sample]):
         rewards = [sample.get_reward_value(args) for sample in samples]
         return len(rewards) == 0 or all(rewards[0] == r for r in rewards)
 
+    def _reward_label(sample: Sample) -> str:
+        # Aborted / None-reward samples have no numeric reward to round; bucket
+        # them under a dedicated label so downstream round() never sees None.
+        reward = sample.get_reward_value(args)
+        if reward is None:
+            return "none"
+        return str(round(reward, 1))
+
     all_sample_groups = group_by(all_samples, lambda s: s.group_index)
     interesting_sample_groups = [g for g in all_sample_groups.values() if _is_zero_std(g)]
 
-    interesting_rewards = [str(round(g[0].get_reward_value(args), 1)) for g in interesting_sample_groups]
+    interesting_rewards = [_reward_label(g[0]) for g in interesting_sample_groups]
 
     return {f"zero_std/count_{reward}": len(items) for reward, items in group_by(interesting_rewards).items()}