Spaces CDN returns SignatureDoesNotMatch

Hi! I’m successfully using DO Spaces with an Elixir libarary to build presigned urls:

signed_url =
  AWSAuth.sign_url(
    @access_key,
    @secret_key,
    "get",
    "https://mybucket.ams3.digitaloceanspaces.com/mybucket/0342cfce-f037-49c6-976f-97b2b8ec6f19.pdf",
    "ams3",
    "s3"
  )

This works fine, but I want to use the CDN to speed up the requests. I tried to add .cdn to the URL:

signed_url =
  AWSAuth.sign_url(
    # ...
    "https://mybucket.ams3.cdn.digitaloceanspaces.com/mybucket/0342cfce-f037-49c6-976f-97b2b8ec6f19.pdf",
    # ...
  )

However, this doesn’t work – I always get the following error:

<Error>
  <Code>SignatureDoesNotMatch</Code>
  <RequestId>tx000000000000049363e85-005bfd730a-b8ca18-ams3a</RequestId>
  <HostId>b8ca18-ams3a-ams3</HostId>
</Error>

I also tried signing the URL without .cdn and just accessing it with .cdn, but got the same error. What am I doing wrong?