diff --git a/src/spikeinterface/core/unitsselectionsorting.py b/src/spikeinterface/core/unitsselectionsorting.py
index fdba9b361a..7a5b9fb1d5 100644
--- a/src/spikeinterface/core/unitsselectionsorting.py
+++ b/src/spikeinterface/core/unitsselectionsorting.py
@@ -61,7 +61,7 @@ def _compute_and_cache_spike_vector(self) -> None:
         )
 
         # check if order is preserved
-        pos = np.searchsorted(self._parent_sorting.unit_ids, self.unit_ids)
+        pos = self._parent_sorting.ids_to_indices(self._unit_ids)
         order_is_preserved = np.all(np.diff(pos) > 0)
 
         if not order_is_preserved: