OpenSSL::PKey::PKeyError when using PEM private key

[adicaff]

stack:
Ruby 2.7.0
rails 6.0.3.5
openssl 2.2.1
jose 1.1.3

While doing JOSE::JWK.from_pem_file(private_key) i got:
Could not parse PKey: no start line (OpenSSL::PKey::PKeyError)

error is in jose-1.1.3/lib/jose/jwk/pem.rb:6:

and trace:

3: from jose-1.1.3/lib/jose/jwk.rb:247:in `from_pem_file'
2: from jose-1.1.3/lib/jose/jwk.rb:237:in `from_pem'
1: from jose-1.1.3/lib/jose/jwk/pem.rb:6:in `from_binary'

The private key is OK, I have tested it using clients software and I can perfectly decrypt payloads. This error comes up only when using the gem on rails project.
Any sights on what can be happening? Any possible solution?
Thanks!