diff --git a/src/spikeinterface/curation/auto_merge.py b/src/spikeinterface/curation/auto_merge.py
index f509ecd6bf..4f84f21d09 100644
--- a/src/spikeinterface/curation/auto_merge.py
+++ b/src/spikeinterface/curation/auto_merge.py
@@ -189,7 +189,12 @@ def get_potential_auto_merge(
 
     # STEP 5 : check if potential merge with CC also have template similarity
     if "template_similarity" in steps:
-        templates = sorting_analyzer.get_extension("templates").get_templates(operator="average")
+        templates_ext = sorting_analyzer.get_extension("templates") or sorting_analyzer.get_extension("fast_templates")
+        assert (
+            templates_ext is not None
+        ), "auto_merge with template_similarity requires a SortingAnalyzer with extension templates or fast_templates"
+
+        templates = templates_ext.get_templates(operator="average")
         templates_diff = compute_templates_diff(
             sorting, templates, num_channels=num_channels, num_shift=num_shift, pair_mask=pair_mask
         )