fatal: bad object xxx when executing git diff

[vduckute1812]

Thank you for your sharing. I met a problem need your help. When I execute this, at

new_files_in_branch=$( git diff \ --name-only \ --diff-filter=AM \ "$BASE_COMMIT" | grep '\.py$' | tr '\n' ' ' )
it raise an error fatal: bad object 170d5fad35bae46edbe5defeb236a0e388c01808 . When I run git branch the values is branch='* (HEAD detached at pull/5/merge)'. Could you please give me some advice about this case?
Thank you so much.

[vduckute1812]

I've known problem, I forgot to fetch branches down locally. Now it work fine.