diff --git a/src/model/example_clusters.py b/src/model/example_clusters.py
index 86abd69..e74b9bf 100644
--- a/src/model/example_clusters.py
+++ b/src/model/example_clusters.py
@@ -28,12 +28,16 @@ class ExampleClusters:
         for sense in model.entry.senses:
             for example in sense.examples:
                 structure = example.get_structure()
-                key = ExampleClusters.get_key(sense, example)
+                cluster = example.get_cluster()
+                
+                if cluster is None:
+                    continue
                 
+                key = ExampleClusters.get_key(sense, example)
                 if key not in self.lists:
                     self.lists[key] = set()
                     
-                self.lists[key].add(example.get_cluster())
+                self.lists[key].add(cluster)
                     
         for key in self.lists.keys():
             self.lists[key] = sorted(self.lists[key], key=lambda x: x)