diff --git a/src/ert/analysis/_es_update.py b/src/ert/analysis/_es_update.py
index 0bc9bf08b1c..00a08421f8e 100644
--- a/src/ert/analysis/_es_update.py
+++ b/src/ert/analysis/_es_update.py
@@ -506,8 +506,9 @@ def _copy_unupdated_parameters(
 
     # Copy the non-updated parameter groups from source to target for each active realization
     for parameter_group in not_updated_parameter_groups:
-        ds = source_ensemble.load_parameters(parameter_group, iens_active_index)
-        target_ensemble.save_parameters(parameter_group, iens_active_index, ds)
+        for realization in iens_active_index:
+            ds = source_ensemble.load_parameters(parameter_group, realization)
+            target_ensemble.save_parameters(parameter_group, realization, ds)
 
 
 def analysis_ES(
diff --git a/src/ert/storage/local_ensemble.py b/src/ert/storage/local_ensemble.py
index b287a45585b..5672b55b80e 100644
--- a/src/ert/storage/local_ensemble.py
+++ b/src/ert/storage/local_ensemble.py
@@ -548,10 +548,10 @@ def _load_single_dataset(
     def _load_dataset(
         self,
         group: str,
-        realizations: Union[int, npt.NDArray[np.int_], None],
+        realizations: Union[int, np.int64, npt.NDArray[np.int_], None],
     ) -> xr.Dataset:
-        if isinstance(realizations, int):
-            return self._load_single_dataset(group, realizations).isel(
+        if isinstance(realizations, (int, np.int64)):
+            return self._load_single_dataset(group, int(realizations)).isel(
                 realizations=0, drop=True
             )
 
@@ -792,7 +792,7 @@ def load_all_gen_kw_data(
     def save_parameters(
         self,
         group: str,
-        realization: Union[int, npt.NDArray[np.int_]],
+        realization: int,
         dataset: xr.Dataset,
     ) -> None:
         """
@@ -820,30 +820,13 @@ def save_parameters(
         if group not in self.experiment.parameter_configuration:
             raise ValueError(f"{group} is not registered to the experiment.")
 
-        # Convert to numpy array if it's an integer
-        realizations: npt.NDArray[np.int_] = (
-            np.array([realization])
-            if isinstance(realization, (int, np.integer))
-            else np.asarray(realization)
-        )
-
-        if realizations.size > 1 and "realizations" not in dataset.dims:
-            raise ValueError(
-                "Dataset must have 'realizations' dimension when saving multiple realizations"
-            )
-
+        path = self._realization_dir(realization) / f"{_escape_filename(group)}.nc"
+        path.parent.mkdir(exist_ok=True)
         if "realizations" in dataset.dims:
-            dataset = dataset.sel(realizations=realizations)
-            for real, data_to_save in dataset.groupby("realizations"):
-                path = self._realization_dir(real) / f"{_escape_filename(group)}.nc"
-                path.parent.mkdir(exist_ok=True)
-                self._storage._to_netcdf_transaction(path, data_to_save)
+            data_to_save = dataset.sel(realizations=[realization])
         else:
-            for real in realizations:
-                path = self._realization_dir(real) / f"{_escape_filename(group)}.nc"
-                path.parent.mkdir(exist_ok=True)
-                data_to_save = dataset.expand_dims(realizations=[real])
-                self._storage._to_netcdf_transaction(path, data_to_save)
+            data_to_save = dataset.expand_dims(realizations=[realization])
+        self._storage._to_netcdf_transaction(path, data_to_save)
 
     @require_write
     def save_response(
diff --git a/tests/ert/ui_tests/cli/test_field_parameter.py b/tests/ert/ui_tests/cli/test_field_parameter.py
index b2262e3eb21..6c1c7cb193a 100644
--- a/tests/ert/ui_tests/cli/test_field_parameter.py
+++ b/tests/ert/ui_tests/cli/test_field_parameter.py
@@ -266,7 +266,8 @@ def test_field_param_update_using_heat_equation_zero_var_params_and_adaptive_loc
             name="prior-zero-var",
         )
         cond["values"][:, :, :5, 0] = 1.0
-        new_prior.save_parameters("COND", range(prior.ensemble_size), cond)
+        for real in range(prior.ensemble_size):
+            new_prior.save_parameters("COND", real, cond)
 
         # Copy responses from existing prior to new prior.
         # Note that we ideally should generate new responses by running the