diff --git a/src/anemoi/models/preprocessing/normalizer.py b/src/anemoi/models/preprocessing/normalizer.py
index 44aae2d..07dfdfc 100644
--- a/src/anemoi/models/preprocessing/normalizer.py
+++ b/src/anemoi/models/preprocessing/normalizer.py
@@ -133,7 +133,7 @@ def _validate_normalization_inputs(self, name_to_index_training_input: dict, min
             ], f"{method} is not a valid normalisation method"
 
     def transform(
-        self, x: torch.Tensor, in_place: bool = True, data_index: Optional[torch.Tensor] = None
+        self, x: torch.Tensor, in_place: bool, data_index: torch.Tensor,
     ) -> torch.Tensor:
         """Normalizes an input tensor x of shape [..., nvars].
 
@@ -157,14 +157,10 @@ def transform(
             _description_
         """
         if not in_place:
-            x = x.clone()
+            x = x.clone()  # TODO: fix this; implement a custom clone() op?
 
-        if data_index is not None:
-            x[..., :] = x[..., :] * self._norm_mul[data_index] + self._norm_add[data_index]
-        elif x.shape[-1] == len(self._input_idx):
-            x[..., :] = x[..., :] * self._norm_mul[self._input_idx] + self._norm_add[self._input_idx]
-        else:
-            x[..., :] = x[..., :] * self._norm_mul + self._norm_add
+        assert data_index is not None  # [Mihai] we require a data_index
+        x[..., :] = x[..., :] * self._norm_mul[data_index] + self._norm_add[data_index]
         return x
 
     def inverse_transform(