X-amz-content-sha256

I am trying to upload an image to a web aws server. The x-amz-content-sha256 is being generated from an empty string. It is not valid. Under the Authorization tab, I included the correct access key, secrety key, and AWS region. I used s3 as the service name and I got the session token from the policy. Anyone know why it is generating this sha256 from an empty string?