diff --git a/matcher-model/src/main/java/matcher/model/classifier/ClassClassifier.java b/matcher-model/src/main/java/matcher/model/classifier/ClassClassifier.java
index a6c558f..3005996 100644
--- a/matcher-model/src/main/java/matcher/model/classifier/ClassClassifier.java
+++ b/matcher-model/src/main/java/matcher/model/classifier/ClassClassifier.java
@@ -126,7 +126,10 @@ public double getScore(ClassInstance clsA, ClassInstance clsB, ClassEnvironment
 	private static AbstractClassifier hierarchySiblings = new AbstractClassifier("hierarchy siblings") {
 		@Override
 		public double getScore(ClassInstance clsA, ClassInstance clsB, ClassEnvironment env) {
-			return ClassifierUtil.compareCounts(clsA.getSuperClass().getChildClasses().size(), clsB.getSuperClass().getChildClasses().size());
+			return ClassifierUtil.compareCounts(
+					clsA.getSuperClass() == null ? 0 : clsA.getSuperClass().getChildClasses().size(),
+					clsB.getSuperClass() == null ? 0 : clsB.getSuperClass().getChildClasses().size()
+			);
 		}
 	};