archive_command fails but works outside of Postgres

I changed my archive_command to the following:
archive_command = 'gsutil cp /storage/postgresql/9.6/main/%p
gs://my_bucket/pg_xlog/'
and it fails, leaving the following in the log:

2017-08-18 18:34:25.057 GMT [1436][0]: [104319] LOG: archive command failed
with exit code 12017-08-18 18:34:25.057 GMT [1436][0]: [104320] DETAIL: The
failed archive command was: gsutil cp
/storage/postgresql/9.6/main/000000010000038B000000D8
gs://my_bucket/pg_xlog/2017-08-18 18:34:25.057 GMT [1436][0]: [104321]
WARNING: archiving transaction log file "000000010000038B000000D8" failed
too many times, will try again later

But the command works when executed manually:
root$ su postgres -c "gsutil cp
/storage/postgresql/9.6/main/000000010000038B000000D8
gs://my_bucket/pg_xlog/"root$ echo $?0
The last command verifies that gsutil indeed exited with 0.

How to best debug this issue?

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