diff --git a/examples/traffic_analysis/inference_example.py b/examples/traffic_analysis/inference_example.py
index 5f06a710..0cbb72c2 100644
--- a/examples/traffic_analysis/inference_example.py
+++ b/examples/traffic_analysis/inference_example.py
@@ -50,8 +50,10 @@ class DetectionsManager:
                     self.counts[zone_out_id][zone_in_id].add(tracker_id)
         if len(detections_all) > 0:
             detections_all.class_id = np.vectorize(
-                lambda x: self.tracker_id_to_zone_id.get(x, -1)
-            )(detections_all.tracker_id)
+                lambda x: self.tracker_id_to_zone_id.get(x, -1))
+            (detections_all.tracker_id)
+        else:
+            detections_all.class_id = np.array([], dtype=int)
         return detections_all[detections_all.class_id != -1]