diff --git a/vlmeval/vlm/idefics.py b/vlmeval/vlm/idefics.py
index 96931d06b..8e4e2c5e9 100644
--- a/vlmeval/vlm/idefics.py
+++ b/vlmeval/vlm/idefics.py
@@ -221,6 +221,8 @@ def build_prompt_mathvista(self, message):
                 for k, v in replace_mapping.items():
                     instruction = instruction.replace(k, v)
                 prompt += instruction.strip()
+        if 'A.' in prompt and 'B.' in prompt:
+            prompt += "\nAnswer with the letter."
         prompt += '<end_of_utterance>\nAssistant:'
         if 'A.' in prompt and 'B.' in prompt:
             prompt += ' Answer:'