Why does git-patch-id(1) sometimes print two lines, one of which has commit = 0000000000000000000000000000000000000000

From: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Date: Feb 16, 2011, 14:56 UTC
Message-ID: <AANLkTimHvA_LsU4YaiDHDKoXnsk5WgGCXgbuSJ99emXw@mail.gmail.com>

This behaves as I'd expect on git.git, i.e. shows a patch id and the commit id:

    $ git show 24231e063f0f003f8ffd7b64c7ba6a0baaaa5283 | git patch-id
    f10c69e0e5b33da206f37bd93639875555ac9b79
24231e063f0f003f8ffd7b64c7ba6a0baaaa5283

But what does this mean, also on git.git:

    $ git show 7d48e9e6f77d336376c1a554eeff0590f77e1ee1 | git patch-id
    4ba8a248731c5fcbd09cacb248d3128e742d1c90
7d48e9e6f77d336376c1a554eeff0590f77e1ee1
    d019b35e0b859cdd6907ee170927de1124c0ed6e
0000000000000000000000000000000000000000

7d48e9e6f77d336376c1a554eeff0590f77e1ee1 is just one of the commits that results in this output:

    $ git log --pretty=format:%H -p | git patch-id | grep
0000000000000000000000000000000000000000
    d019b35e0b859cdd6907ee170927de1124c0ed6e
0000000000000000000000000000000000000000
    3b23a2a11055aef557369971e825010879a8c4d7
0000000000000000000000000000000000000000
    d498fbbad6f1374d952925df699da237c3e8f2df
0000000000000000000000000000000000000000
    b0c930dc1926ffae9cca022797856762fa908be6
0000000000000000000000000000000000000000

And on another repository where I'm dealing with this I have a bunch more of them.

Why are they there and what do they mean? Maybe it's a bug, or something I can explain in the manual page.