NilClass error when signing xml

[alanBAguero]

This is the script I used to sign:

unsigned_xml = File.read("/home/path/unsigned.xml"); 0
private_key = OpenSSL::PKey::RSA.new(File.read("/home/path/certificates/alice.key")); 0
certificate = OpenSSL::X509::Certificate.new(File.read("/home/path/alice.crt")); 0
unsigned_document = Xmldsig::SignedDocument.new(unsigned_xml, id_attr: "Id"); 0
signed_xml = unsigned_document.sign(private_key)

But I'm getting a NoMethodError (undefined method remove' for nil:NilClass)`.
Do you know what might be the issue? Sorry, this is my first time working with XMLs and I'm kind of stuck
with this issue. If you could please help me, I'd appreciate it a lot!

Here's a gist with unsigned xml, certificate and private key: https://gist.github.com/alanBAguero/62d77ee45f14f15a16b376e08118ee18

[alanBAguero]

The xml has the following structure:

- rDE
  - dVerFor
  - DE, Id="some-id"  # I need to sign these fields
  - Signature # "some-id" is specified on URI attribute of Reference tag

[Guistoff081]

I am stuck with the same error:

  unsigned_document = Xmldsig::SignedDocument.new(xml_document.doc, id_attr: 'Id')
  signed_xml = unsigned_document.sign(@private_key)

raises NoMethodError:
undefined method `remove' for nil:NilClass