Docusign JWT Token returns no_valid_key_or_signatures (node)

I haven't been able to authenticate with DocuSign's API using nodejs. These are the steps I have followed;

1. I got the DocuSign code example running with JWT grant for node.
2. I generated an Integration Key in the Sandbox environment
3. I stored my private RSA Key and it's properly formed (it doesn't have trailing spaces).

But I keep getting this error (I also tried generating new integration keys but I get the same result)

 API problem: Status code 400, message body:
 {
    "error": "invalid_grant",
    "error_description": "no_valid_keys_or_signatures"
 }