diff --git a/src/spikeinterface/widgets/unit_summary.py b/src/spikeinterface/widgets/unit_summary.py
index fd7dafd5d6..6c39c902eb 100644
--- a/src/spikeinterface/widgets/unit_summary.py
+++ b/src/spikeinterface/widgets/unit_summary.py
@@ -85,6 +85,7 @@ def plot_matplotlib(self, data_plot, **backend_kwargs):
         unit_colors = dp.unit_colors
         sparsity = dp.sparsity
 
+        # defaultdict returns empty dict if key not found in subwidget_kwargs
         subwidget_kwargs = defaultdict(lambda: dict(), dp.subwidget_kwargs)
         unitlocationswidget_kwargs = subwidget_kwargs["unit_locations"]
         unitwaveformswidget_kwargs = subwidget_kwargs["unit_waveforms"]