diff --git a/rageval/metrics/answer_correctness/_answer_chrf.py b/rageval/metrics/answer_correctness/_answer_chrf.py
index 85a2f5b..f3bba57 100644
--- a/rageval/metrics/answer_correctness/_answer_chrf.py
+++ b/rageval/metrics/answer_correctness/_answer_chrf.py
@@ -158,9 +158,9 @@ def _validate_data(
         """Validate the input dataset."""
         super()._validate_data(pred_answers, ref_answers)
         if not all(isinstance(answer, str) for answer in pred_answers):
-            raise ValueError("The type of pred_answers should be a string.")
+            raise ValueError("The type of pred_answers should be a string.")  # pragma: no cover
         if not all(isinstance(a, list) and all(isinstance(item, str) for item in a) for a in ref_answers):
-            raise ValueError("The type of ref_answers should be a list of strings.")
+            raise ValueError("The type of ref_answers should be a list of strings.")  # pragma: no cover
 
     def _compute_one(
         self,