diff --git a/src/main/java/io/anserini/util/ExtractDocumentLengths.java b/src/main/java/io/anserini/util/ExtractDocumentLengths.java
index 232f2ee99b..ca8820672f 100644
--- a/src/main/java/io/anserini/util/ExtractDocumentLengths.java
+++ b/src/main/java/io/anserini/util/ExtractDocumentLengths.java
@@ -17,6 +17,7 @@
 package io.anserini.util;
 
 import io.anserini.index.IndexArgs;
+import io.anserini.index.IndexReaderUtils;
 import io.anserini.index.NotStoredException;
 import org.apache.lucene.index.DirectoryReader;
 import org.apache.lucene.index.IndexReader;
@@ -72,14 +73,15 @@ public static void main(String[] args) throws Exception {
     long lossyTotalTerms = 0;
     long exactTotalTerms = 0;
 
-    out.println("docid\tdoc_length\tunique_term_count\tlossy_doc_length\tlossy_unique_term_count");
+    out.println("internal_docid\texternal_docid\tdoc_length\tunique_term_count\tlossy_doc_length\tlossy_unique_term_count");
     for (int i = 0; i < numDocs; i++) {
       Terms terms = reader.getTermVector(i, IndexArgs.CONTENTS);
       if (terms == null) {
         // It could be the case that TermVectors weren't stored when constructing the index, or we're just missing a
         // TermVector for a zero-length document. Warn, but don't throw exception.
-        System.err.println(String.format("Warning: TermVector not available for docid %d.", i));
-        out.println(String.format("%d\t0\t0\t0\t0", i));
+        String external_did = IndexReaderUtils.convertLuceneDocidToDocid(reader, i);
+        System.err.println(String.format("Warning: TermVector not available for docid %s.", external_did));
+        out.println(String.format("%d\t%s\t0\t0\t0\t0", i, external_did));
         continue;
       }
 
@@ -90,7 +92,8 @@ public static void main(String[] args) throws Exception {
       // See https://github.com/apache/lucene-solr/blob/master/lucene/core/src/java/org/apache/lucene/search/similarities/BM25Similarity.java
       int lossyDoclength = SmallFloat.byte4ToInt(SmallFloat.intToByte4((int) exactDoclength));
       int lossyTermCount = SmallFloat.byte4ToInt(SmallFloat.intToByte4((int) exactTermCount));
-      out.println(String.format("%d\t%d\t%d\t%d\t%d", i, exactDoclength, exactTermCount, lossyDoclength, lossyTermCount));
+      out.println(String.format("%d\t%s\t%d\t%d\t%d\t%d", i, IndexReaderUtils.convertLuceneDocidToDocid(reader, i),
+              exactDoclength, exactTermCount, lossyDoclength, lossyTermCount));
       lossyTotalTerms += lossyDoclength;
       exactTotalTerms += exactDoclength;
     }
diff --git a/src/test/java/io/anserini/util/ExtractDocumentLengthsTest.java b/src/test/java/io/anserini/util/ExtractDocumentLengthsTest.java
index bbdd25daa7..882fee79f2 100644
--- a/src/test/java/io/anserini/util/ExtractDocumentLengthsTest.java
+++ b/src/test/java/io/anserini/util/ExtractDocumentLengthsTest.java
@@ -72,9 +72,9 @@ public void test() throws Exception {
 
     List<String> lines = Files.readAllLines(Paths.get(randomFileName));
     assertEquals(5, lines.size());
-    assertEquals("0\t8\t5\t8\t5", lines.get(1));
-    assertEquals("1\t2\t2\t2\t2", lines.get(2));
-    assertEquals("2\t2\t2\t2\t2", lines.get(3));
-    assertEquals("3\t0\t0\t0\t0", lines.get(4));
+    assertEquals("0\tdoc1\t8\t5\t8\t5", lines.get(1));
+    assertEquals("1\tdoc2\t2\t2\t2\t2", lines.get(2));
+    assertEquals("2\tdoc3\t2\t2\t2\t2", lines.get(3));
+    assertEquals("3\tdoc4\t0\t0\t0\t0", lines.get(4));
   }
 }