diff --git a/lucene/core/src/java/org/apache/lucene/util/RamUsageEstimator.java b/lucene/core/src/java/org/apache/lucene/util/RamUsageEstimator.java
index 4176539f155..a2dc7506023 100644
--- a/lucene/core/src/java/org/apache/lucene/util/RamUsageEstimator.java
+++ b/lucene/core/src/java/org/apache/lucene/util/RamUsageEstimator.java
@@ -420,6 +420,7 @@ public final class RamUsageEstimator {
       seen.add(ob);
 
       final Class<?> obClazz = ob.getClass();
+      assert obClazz != null : "jvm bug detected (Object.getClass() == null). please report this to your vendor";
       if (obClazz.isArray()) {
         /*
          * Consider an array, possibly of primitive types. Push any of its references to