diff --git a/antarest/study/storage/variantstudy/model/command/create_st_storage.py b/antarest/study/storage/variantstudy/model/command/create_st_storage.py
index 0b008b61bf..b029336fab 100644
--- a/antarest/study/storage/variantstudy/model/command/create_st_storage.py
+++ b/antarest/study/storage/variantstudy/model/command/create_st_storage.py
@@ -141,7 +141,7 @@ def register_matrix(
                 "inflows": constants.get_st_storage_inflows,
             }
             method = methods[field.name]
-            return cast(str, method())
+            return method()
         if isinstance(v, str):
             # Check the matrix link
             return validate_matrix(v, values)