diff --git a/src/spikeinterface/extractors/phykilosortextractors.py b/src/spikeinterface/extractors/phykilosortextractors.py
index 7ffbc166de..a75ce97198 100644
--- a/src/spikeinterface/extractors/phykilosortextractors.py
+++ b/src/spikeinterface/extractors/phykilosortextractors.py
@@ -159,7 +159,8 @@ def __init__(
                 self.set_property(key="original_cluster_id", values=cluster_info[prop_name])
             elif prop_name == "group":
                 # rename group property to 'quality'
-                self.set_property(key="quality", values=cluster_info[prop_name])
+                values = cluster_info[prop_name].values.astype("str")
+                self.set_property(key="quality", values=values)
             else:
                 if load_all_cluster_properties:
                     # pandas loads strings with empty values as objects with NaNs