Fail to change user-defined metadata via S3

I am using s3cmd to interact with my objects that carry some user-defined metadata. However, I don't seem to be able to update them after they have been uploaded. Using s3cmd modify command, I always get HTTP 403 (Forbidden) with code 'SignatureDoesNotMatch'. Any idea why?

I used the following command to update the metadata:

/usr/bin/s3cmd -d --add-header="x-amz-meta-play:world" modify s3://play-bucket/9G

It is interesting to note that even without the "--add-header" part, it still fails with the same code 'SignatureDoesNotMatch'.