diff --git a/grader/langchain.py b/grader/langchain.py
index 60b9256..846b557 100644
--- a/grader/langchain.py
+++ b/grader/langchain.py
@@ -4,9 +4,13 @@
 from enum import Enum
 from typing import List, Optional
 
-from pydantic import BaseModel, Field, model_validator
+from pydantic import BaseModel, Field
 
-from .exceptions import IncorrectResponseValueError, ResponseFailedError
+from .exceptions import (
+    IncorrectResponseValueError,
+    InvalidResponseStructureError,
+    ResponseFailedError,
+)
 
 
 class LCRequestMetaData(BaseModel):
@@ -41,13 +45,6 @@ class LCChatMemory(BaseModel):
 
     messages: List[LCMessage]
 
-    @model_validator(mode="after")
-    def validate_messages(self) -> "LCChatMemory":
-        """Validate that chat memory contains at least 2 dicts"""
-        if len(self.messages) < 2:
-            raise ValueError("messages must contain at least 2 objects")
-        return self
-
 
 class LCBody(BaseModel):
     """LangChain body"""
@@ -82,6 +79,9 @@ def validate_is_base64_encoded(self):
     def validate_prompt_sequence(self):
         """Validate that the prompt sequence begins with a human message and ends with an ai message"""
         messages = self.body.chat_memory.messages
+        if len(messages) < 2:
+            raise InvalidResponseStructureError("messages must contain at least 2 objects")
+
         prompt_1 = messages[0]
         if prompt_1.type != MessageType.human:
             raise IncorrectResponseValueError(