Index: ext/fts3/fts3_write.c
==================================================================
--- ext/fts3/fts3_write.c
+++ ext/fts3/fts3_write.c
@@ -3634,10 +3634,11 @@
   sqlite3_int64 iAbsLevel;        /* Absolute level of input segments */
   int iIdx;                       /* Index of *output* segment in iAbsLevel+1 */
   sqlite3_int64 iStart;           /* Block number of first allocated block */
   sqlite3_int64 iEnd;             /* Block number of last allocated block */
   sqlite3_int64 nLeafData;        /* Bytes of leaf page data so far */
+  u8 bNoLeafData;                 /* If true, store 0 for segment size */
   NodeWriter aNodeWriter[FTS_MAX_APPENDABLE_HEIGHT];
 };
 
 /*
 ** An object of the following type is used to read data from a single
@@ -4072,11 +4073,11 @@
         pWriter->iAbsLevel+1,               /* level */
         pWriter->iIdx,                      /* idx */
         pWriter->iStart,                    /* start_block */
         pWriter->aNodeWriter[0].iBlock,     /* leaves_end_block */
         pWriter->iEnd,                      /* end_block */
-        pWriter->nLeafData,                 /* end_block */
+        (pWriter->bNoLeafData==0 ? pWriter->nLeafData : 0),   /* end_block */
         pRoot->block.a, pRoot->block.n      /* root */
     );
   }
   sqlite3_free(pRoot->block.a);
   sqlite3_free(pRoot->key.a);
@@ -4178,10 +4179,11 @@
       iLeafEnd = sqlite3_column_int64(pSelect, 2);
       fts3ReadEndBlockField(pSelect, 3, &iEnd, &pWriter->nLeafData);
       if( pWriter->nLeafData<0 ){
         pWriter->nLeafData = pWriter->nLeafData * -1;
       }
+      pWriter->bNoLeafData = (pWriter->nLeafData==0);
       nRoot = sqlite3_column_bytes(pSelect, 4);
       aRoot = sqlite3_column_blob(pSelect, 4);
     }else{
       return sqlite3_reset(pSelect);
     }
@@ -4912,11 +4914,11 @@
 
       if( nSeg!=0 ){
         pWriter->nLeafData = pWriter->nLeafData * -1;
       }
       fts3IncrmergeRelease(p, pWriter, &rc);
-      if( nSeg==0 ){
+      if( nSeg==0 && pWriter->bNoLeafData==0 ){
         fts3PromoteSegments(p, iAbsLevel+1, pWriter->nLeafData);
       }
     }
 
     sqlite3Fts3SegReaderFinish(pCsr);

Index: test/fts4growth.test
==================================================================
--- test/fts4growth.test
+++ test/fts4growth.test
@@ -364,23 +364,74 @@
   SELECT level, idx, end_block FROM x5_segdir;
 } {
   0 1 {1329 8297} 0 0 {1320 28009} 0 2 {1449 118006}
 }
 
-#do_test 3.2 {
-  #t1_to_x2
-  #execsql {SELECT level, count(*) FROM x2_segdir GROUP BY level}
-#} {0 13 1 15 2 5}
+#--------------------------------------------------------------------------
+# Ensure that if part of an incremental merge is performed by an old
+# version that does not support storing segment sizes in the end_block
+# field, no size is stored in the final segment (as it would be incorrect).
+#
+do_execsql_test 7.1 {
+  CREATE VIRTUAL TABLE x6 USING fts4;
+  INSERT INTO x6 SELECT words FROM t1;
+  INSERT INTO x6 SELECT words FROM t1;
+  INSERT INTO x6 SELECT words FROM t1;
+  INSERT INTO x6 SELECT words FROM t1;
+  INSERT INTO x6 SELECT words FROM t1;
+  INSERT INTO x6 SELECT words FROM t1;
+  SELECT level, idx, end_block FROM x6_segdir;
+} {
+  0 0 {118 117483} 0 1 {238 118006} 0 2 {358 118006} 
+  0 3 {478 118006} 0 4 {598 118006} 0 5 {718 118006}
+}
+
+do_execsql_test 7.2 {
+  INSERT INTO x6(x6) VALUES('merge=25,4');
+  SELECT level, idx, end_block FROM x6_segdir;
+} {
+  0 0 {118 117483} 0 1 {238 118006} 0 2 {358 118006} 
+  0 3 {478 118006} 0 4 {598 118006} 0 5 {718 118006}
+  1 0 {16014 -51226}
+}
+
+do_execsql_test 7.3 {
+  UPDATE x6_segdir SET end_block = first(end_block) WHERE level=1;
+  SELECT level, idx, end_block FROM x6_segdir;
+} {
+  0 0 {118 117483} 0 1 {238 118006} 0 2 {358 118006} 
+  0 3 {478 118006} 0 4 {598 118006} 0 5 {718 118006}
+  1 0 16014
+}
+
+do_execsql_test 7.4 {
+  INSERT INTO x6(x6) VALUES('merge=25,4');
+  SELECT level, idx, end_block FROM x6_segdir;
+} {
+  0 0 {118 117483} 0 1 {238 118006} 0 2 {358 118006} 
+  0 3 {478 118006} 0 4 {598 118006} 0 5 {718 118006}
+  1 0 16014
+}
+
+do_execsql_test 7.5 {
+  INSERT INTO x6(x6) VALUES('merge=2500,4');
+  SELECT level, idx, end_block FROM x6_segdir;
+} {
+  0 0 {598 118006} 0 1 {718 118006} 1 0 16014
+}
+
+do_execsql_test 7.6 {
+  INSERT INTO x6(x6) VALUES('merge=2500,2');
+  SELECT level, idx, start_block, leaves_end_block, end_block FROM x6_segdir;
+} {
+  2 0 23695 24147 {41262 633507}
+}
+
+do_execsql_test 7.7 {
+  SELECT sum(length(block)) FROM x6_segments 
+  WHERE blockid BETWEEN 23695 AND 24147
+} {633507}
 
-#proc second {x} { lindex $x 1 }
-#db func second second
-#for {set i 0} {$i <1000} {incr i} {
-#  t1_to_x2
-#  db eval {
-#    SELECT level, group_concat( second(end_block), ' ' ) AS c FROM x2_segdir GROUP BY level;
-#  } {
-#    puts "$i.$level: $c"
-#  }
-#}
+
 
 finish_test