diff --git a/Documentation/technical/index-format.txt b/Documentation/technical/index-format.txt
index 8930b3fabc..9d25b30178 100644
--- a/Documentation/technical/index-format.txt
+++ b/Documentation/technical/index-format.txt
@@ -113,9 +113,22 @@ GIT index format
     are encoded in 7-bit ASCII and the encoding cannot contain a NUL
     byte (iow, this is a UNIX pathname).
 
+  (Version 4) In version 4, the entry path name is prefix-compressed
+    relative to the path name for the previous entry (the very first
+    entry is encoded as if the path name for the previous entry is an
+    empty string).  At the beginning of an entry, an integer N in the
+    variable width encoding (the same encoding as the offset is encoded
+    for OFS_DELTA pack entries; see pack-format.txt) is stored, followed
+    by a NUL-terminated string S.  Removing N bytes from the end of the
+    path name for the previous entry, and replacing it with the string S
+    yields the path name for this entry.
+
   1-8 nul bytes as necessary to pad the entry to a multiple of eight bytes
   while keeping the name NUL-terminated.
 
+  (Version 4) In version 4, the padding after the pathname does not
+  exist.
+
 == Extensions
 
 === Cached tree
diff --git a/unpack-trees.c b/unpack-trees.c
index 36523da22a..1d7393d84c 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -1027,6 +1027,7 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 	o->result.initialized = 1;
 	o->result.timestamp.sec = o->src_index->timestamp.sec;
 	o->result.timestamp.nsec = o->src_index->timestamp.nsec;
+	o->result.version = o->src_index->version;
 	o->merge_size = len;
 	mark_all_ce_unused(o->src_index);