diff --git a/antarest/study/storage/rawstudy/model/filesystem/matrix/date_serializer.py b/antarest/study/storage/rawstudy/model/filesystem/matrix/date_serializer.py
index 4f2c6213cc..2b8c431085 100644
--- a/antarest/study/storage/rawstudy/model/filesystem/matrix/date_serializer.py
+++ b/antarest/study/storage/rawstudy/model/filesystem/matrix/date_serializer.py
@@ -110,7 +110,7 @@ def extract_date(self, df: pd.DataFrame) -> Tuple[pd.Index[str], pd.DataFrame]:
         to_remove = cast(Sequence[Hashable], df.columns[0:5])
         body = df.drop(to_remove, axis=1)
 
-        return cast(pd.Index[str], pd.Index(date)), body
+        return pd.Index(date), body  # type: ignore
 
 
 class DailyMatrixSerializer(IDateMatrixSerializer):
@@ -148,7 +148,7 @@ def extract_date(self, df: pd.DataFrame) -> Tuple[pd.Index[str], pd.DataFrame]:
         to_remove = cast(Sequence[Hashable], df.columns[0:4])
         body = df.drop(to_remove, axis=1)
 
-        return cast(pd.Index[str], pd.Index(date)), body
+        return pd.Index(date), body  # type: ignore
 
 
 class WeeklyMatrixSerializer(IDateMatrixSerializer):