diff --git a/lucene/src/java/org/apache/lucene/index/SegmentReader.java b/lucene/src/java/org/apache/lucene/index/SegmentReader.java
index 95e4999e736..3b71ab64c34 100644
--- a/lucene/src/java/org/apache/lucene/index/SegmentReader.java
+++ b/lucene/src/java/org/apache/lucene/index/SegmentReader.java
@@ -750,6 +750,8 @@ public class SegmentReader extends IndexReader implements Cloneable {
     if (deletedDocsDirty) {               // re-write deleted
       si.advanceDelGen();
 
+      assert deletedDocs.length() == si.docCount;
+
       // We can write directly to the actual name (vs to a
       // .tmp & renaming it) because the file is not live
       // until segments file is written: