diff --git a/src/spikeinterface/curation/curation_format.py b/src/spikeinterface/curation/curation_format.py
index 5f85538b08..80f251ca43 100644
--- a/src/spikeinterface/curation/curation_format.py
+++ b/src/spikeinterface/curation/curation_format.py
@@ -45,12 +45,16 @@ def validate_curation_dict(curation_dict):
         if not removed_units_set.issubset(unit_set):
             raise ValueError("Curation format: some removed units are not in the unit list")
 
+    for group in curation_dict["merge_unit_groups"]:
+        if len(group) < 2:
+            raise ValueError("Curation format: 'merge_unit_groups' must be list of list with at least 2 elements")
+
     all_merging_groups = [set(group) for group in curation_dict["merge_unit_groups"]]
     for gp_1, gp_2 in combinations(all_merging_groups, 2):
         if len(gp_1.intersection(gp_2)) != 0:
-            raise ValueError("Some units belong to multiple merge groups")
+            raise ValueError("Curation format: some units belong to multiple merge groups")
     if len(removed_units_set.intersection(merged_units_set)) != 0:
-        raise ValueError("Some units were merged and deleted")
+        raise ValueError("Curation format: some units were merged and deleted")
 
     # Check the labels exclusivity
     for lbl in curation_dict["manual_labels"]:
@@ -238,7 +242,7 @@ def apply_curation_labels(sorting, new_unit_ids, curation_dict):
         all_values = np.zeros(sorting.unit_ids.size, dtype=values.dtype)
         for unit_ind, unit_id in enumerate(sorting.unit_ids):
             if unit_id not in new_unit_ids:
-                ind = curation_dict["unit_ids"].index(unit_id)
+                ind = list(curation_dict["unit_ids"]).index(unit_id)
                 all_values[unit_ind] = values[ind]
         sorting.set_property(key, all_values)
 
@@ -253,7 +257,7 @@ def apply_curation_labels(sorting, new_unit_ids, curation_dict):
                         group_values.append(value)
                 if len(set(group_values)) == 1:
                     # all group has the same label or empty
-                    sorting.set_property(key, values=group_values, ids=[new_unit_id])
+                    sorting.set_property(key, values=group_values[:1], ids=[new_unit_id])
             else:
 
                 for key in label_def["label_options"]:
@@ -339,18 +343,22 @@ def apply_curation(
 
     elif isinstance(sorting_or_analyzer, SortingAnalyzer):
         analyzer = sorting_or_analyzer
-        analyzer = analyzer.remove_units(curation_dict["removed_units"])
-        analyzer, new_unit_ids = analyzer.merge_units(
-            curation_dict["merge_unit_groups"],
-            censor_ms=censor_ms,
-            merging_mode=merging_mode,
-            sparsity_overlap=sparsity_overlap,
-            new_id_strategy=new_id_strategy,
-            return_new_unit_ids=True,
-            format="memory",
-            verbose=verbose,
-            **job_kwargs,
-        )
+        if len(curation_dict["removed_units"]) > 0:
+            analyzer = analyzer.remove_units(curation_dict["removed_units"])
+        if len(curation_dict["merge_unit_groups"]) > 0:
+            analyzer, new_unit_ids = analyzer.merge_units(
+                curation_dict["merge_unit_groups"],
+                censor_ms=censor_ms,
+                merging_mode=merging_mode,
+                sparsity_overlap=sparsity_overlap,
+                new_id_strategy=new_id_strategy,
+                return_new_unit_ids=True,
+                format="memory",
+                verbose=verbose,
+                **job_kwargs,
+            )
+        else:
+            new_unit_ids = []
         apply_curation_labels(analyzer.sorting, new_unit_ids, curation_dict)
         return analyzer
     else: