S3 cp AccessDenied from AWS cli with root keys

I have the AWS cli installed on an EC2 instance, and I configured it by running aws configure and giving it my AWSAccessKeyId and AWSSecretKey keys so if I run the command aws s3 ls it returns the name of my S3 bucket (call it "mybucket").

But, if I then try aws s3 cp localfolder/ s3://mybucket/ --recursive I get an error that looks like

A client error (AccessDenied) occurred when calling the CreateMultipartUpload operation: Anonymous users cannot initiate multipart uploads.  Please authenticate.

I thought that by running aws configure that I was effectively giving the aws cli everything it needs to authenticate? Is there something I am missing regarding copying to an S3 bucket as opposed to listing them?