What can cause empty GIT_AUTHOR_NAME for 'git filter-branch --tree-filter' on Solaris?

The filter-branch command, the contents of ~/.gitconfig and the tree
are the same.
The command succeeds on cygwin, but fails on Solaris due to
unset GIT_AUTHOR_NAME and GIT_COMMITTER_NAME :

$ git filter-branch --tree-filter "env | grep GIT_ ; $CMD" b416b9bfc5e71531f2f05af4c396bb0ba7560741..HEAD
Rewrite 214efc6eec82b015aefe23b2280979f05b351396 (1/16)GIT_DIR=/home/tester/.ilya/builds/makepkg.rap_0.1-1_sparc.XXXXXX/src/rap/.git
GIT_INDEX_FILE=/home/tester/.ilya/builds/makepkg.rap_0.1-1_sparc.XXXXXX/src/rap/.git-rewrite/t/../index
GIT_WORK_TREE=.
GIT_AUTHOR_NAME=
GIT_COMMITTER_NAME=
GIT_COMMIT=214efc6eec82b015aefe23b2280979f05b351396
fatal: empty ident  [off-list ref] not allowed
could not write rewritten commit

If I explicitly set these 2 variables, filter-branch succeeds, but
other commit attributes like commit date aren't preserved.

I use git 1.7.6, from sunfreeware.

I hope there is some simple thing that needs to be configured.