Diff-tree does not work for initial commit

Hi, It looks like I found a bug in git.

The problem: In my script I need to know what files were modified by given commit. I use diff-tree for it. Although it works for most cases, for initial commit it does not. Here is a sequence of actions.

anatol:~ $ mkdir mkdir initialcommitissue
anatol:~ $ cd initialcommitissue/
anatol:initialcommitissue $ git init
Initialized empty Git repository in /home/anatol/initialcommitissue/.git/
anatol:initialcommitissue $ echo "First commit" > 1.txt
anatol:initialcommitissue $ git add 1.txt
anatol:initialcommitissue $ git commit -m "First commit"
Created initial commit 31ccc6a: First commit
 1 files changed, 1 insertions(+), 0 deletions(-)
 create mode 100644 1.txt
anatol:initialcommitissue $ git diff-tree HEAD     <<<<< PROBLEM IS HERE
anatol:initialcommitissue $ echo "Second commit" > 2.txt
anatol:initialcommitissue $ git add 2.txt
anatol:initialcommitissue $ git commit -m "Second commit"
Created commit 51e8bcb: Second commit
 1 files changed, 1 insertions(+), 0 deletions(-)
 create mode 100644 2.txt
anatol:initialcommitissue $ git diff-tree HEAD
51e8bcbb739fc8329fc092db7a84b02bbc64feb2
:000000 100644 0000000000000000000000000000000000000000
c133ee6afb86d836ae607cc12e7b7b42242aa5fa A	2.txt

so git diff-tree HEAD works fine but git diff-tree HEAD~1 does not. I guess in sake of consistency it should show all changed files in initial commit.