[BUG?] git log does not decorate when custom format is used

Hi,

is it a bug?

When I use something like:
git log --pretty=format:'%H %s' --decorate
I do not get the decoration.
While 
git log --pretty=oneline --decorate
does decorate.

I'm using:
git version 1.5.6.1

Cheers,
Michael