diff --git a/server/src/main/java/org/elasticsearch/action/admin/indices/diskusage/IndexDiskUsageAnalyzer.java b/server/src/main/java/org/elasticsearch/action/admin/indices/diskusage/IndexDiskUsageAnalyzer.java
index 3dadabfa3f519..6587bf27f604a 100644
--- a/server/src/main/java/org/elasticsearch/action/admin/indices/diskusage/IndexDiskUsageAnalyzer.java
+++ b/server/src/main/java/org/elasticsearch/action/admin/indices/diskusage/IndexDiskUsageAnalyzer.java
@@ -314,6 +314,7 @@ private static BlockTermState getBlockTermState(TermsEnum termsEnum, BytesRef te
             if (termState instanceof final Lucene50PostingsFormat.IntBlockTermState blockTermState) {
                 return new BlockTermState(blockTermState.docStartFP, blockTermState.posStartFP, blockTermState.payStartFP);
             }
+            assert false : "unsupported postings format: " + termState;
         }
         return null;
     }