Clarifying "invalid tag signature file" error message from git filter-branch (and others)

What do the errors "error: char88: malformed tagger field" and "fatal:
invalid tag signature file" and "Could not create new tag object for
FINDUTILS-4_1-10" signify in the session below?

Are any of those errors correctable (I can re-run the tree rewrite
script as many times as needed, I'm just using it on a test repository
for now).

The script I'm invoking calls git filter-branch; I've attached the script.

$ bash ~/source/GNU/findutils/logrewrite.sh
fatal: Not a git repository
Rewrite 9afb13614381c4bfa22f515c18cbfe6f54798330 (1587/1587)
Ref 'refs/heads/master' was rewritten
FINDUTILS-4_1-10 -> FINDUTILS-4_1-10
(ce25eb352de8dc53a9a7805ba9efc1c9215d28c2 ->
79221244237bc408ced0c37f3c3476d53e665801)
error: char88: malformed tagger field
fatal: invalid tag signature file
Could not create new tag object for FINDUTILS-4_1-10
$

Thanks,
James.