diff --git a/mm_agents/agent.py b/mm_agents/agent.py
index 5ed3d27..a4b1b3a 100644
--- a/mm_agents/agent.py
+++ b/mm_agents/agent.py
@@ -264,9 +264,14 @@ class PromptAgent:
             , "The number of observations and actions should be the same."
 
         if len(self.observations) > self.max_trajectory_length:
-            _observations = self.observations[-self.max_trajectory_length:]
-            _actions = self.actions[-self.max_trajectory_length:]
-            _thoughts = self.thoughts[-self.max_trajectory_length:]
+            if self.max_trajectory_length == 0:
+                _observations = []
+                _actions = []
+                _thoughts = []
+            else:
+                _observations = self.observations[-self.max_trajectory_length:]
+                _actions = self.actions[-self.max_trajectory_length:]
+                _thoughts = self.thoughts[-self.max_trajectory_length:]
         else:
             _observations = self.observations
             _actions = self.actions