Package artifact not finding my s3 bucket

I’ve set my serverless.yml with:

package:
  artifact: s3://_a_private_bucket_/_sls_app_.zip

as shown in Serverless Framework Guide - AWS Lambda Guide - Packaging

But when I do sls deploy I get this flabgastering error:

 Exception -----------------------------------------------

  [OperationalError: ENOENT: no such file or directory, open 's3://_a_private_bucket_/_sls_app_.zip'] {

however, I have no problems to access it with aws s3 ls s3://_a_private_bucket_/_sls_app_.zip.

Besides, I’ve been able to deploy other similar projects using my S3.

I’m trying to deploy a project whose artifact is bigger than 260 Mb uncompressed (so, could it not work even if I get S3 to work?).