diff --git a/lockfile.h b/lockfile.h
index f401c979f0..35403ccc0d 100644
--- a/lockfile.h
+++ b/lockfile.h
@@ -263,8 +263,8 @@ static inline int close_lock_file_gently(struct lock_file *lk)
  *   nobody else) to inspect the contents you wrote, while still
  *   holding the lock yourself.
  *
- * * `reopen_lock_file()` to reopen the lockfile. Make further updates
- *   to the contents.
+ * * `reopen_lock_file()` to reopen the lockfile, truncating the existing
+ *   contents. Write out the new contents.
  *
  * * `commit_lock_file()` to make the final version permanent.
  */
diff --git a/t/t0090-cache-tree.sh b/t/t0090-cache-tree.sh
index 0c61268fd2..596e269394 100755
--- a/t/t0090-cache-tree.sh
+++ b/t/t0090-cache-tree.sh
@@ -161,6 +161,24 @@ test_expect_success PERL 'commit --interactive gives cache-tree on partial commi
 	test_cache_tree
 '
 
+test_expect_success PERL 'commit -p with shrinking cache-tree' '
+	mkdir -p deep/subdir &&
+	echo content >deep/subdir/file &&
+	git add deep &&
+	git commit -m add &&
+	git rm -r deep &&
+
+	before=$(wc -c <.git/index) &&
+	git commit -m delete -p &&
+	after=$(wc -c <.git/index) &&
+
+	# double check that the index shrank
+	test $before -gt $after &&
+
+	# and that our index was not corrupted
+	git fsck
+'
+
 test_expect_success 'commit in child dir has cache-tree' '
 	mkdir dir &&
 	>dir/child.t &&
diff --git a/tempfile.c b/tempfile.c
index 139ecd97f8..d43ad8c191 100644
--- a/tempfile.c
+++ b/tempfile.c
@@ -279,7 +279,7 @@ int reopen_tempfile(struct tempfile *tempfile)
 		BUG("reopen_tempfile called for an inactive object");
 	if (0 <= tempfile->fd)
 		BUG("reopen_tempfile called for an open object");
-	tempfile->fd = open(tempfile->filename.buf, O_WRONLY);
+	tempfile->fd = open(tempfile->filename.buf, O_WRONLY|O_TRUNC);
 	return tempfile->fd;
 }
 
diff --git a/tempfile.h b/tempfile.h
index 8959c5f1b5..b1b085540b 100644
--- a/tempfile.h
+++ b/tempfile.h
@@ -235,8 +235,8 @@ extern int close_tempfile_gently(struct tempfile *tempfile);
  *   it (and nobody else) to inspect or even modify the file's
  *   contents.
  *
- * * `reopen_tempfile()` to reopen the temporary file. Make further
- *   updates to the contents.
+ * * `reopen_tempfile()` to reopen the temporary file, truncating the existing
+ *   contents. Write out the new contents.
  *
  * * `rename_tempfile()` to move the file to its permanent location.
  */