Auth JWT not authenticating - returning a 'sub' claim

Following up on yesterday's succesful ask.  I have a, hopefully, straightforward question while setting this up.

Code:

from boxsdk import JWTAuth

auth = JWTAuth(
    client_id='*****',
    client_secret='*****',
    enterprise_id=99999,
    jwt_key_id='****',
    rsa_private_key_file_sys_path='mykey.pem',
#     store_tokens="",
)

access_token = auth.authenticate_instance()


auth_url, csrf_token = auth.get_authorization_url('https://www.my.redirect.com')

I am using the following code above with the appropriate id, secret etc - but it keeps breaking at the auth.athenticate_instance()  and returns:

BoxOAuthException: 
Message: {"error":"invalid_grant","error_description":"Please check the 'sub' claim."}
Status: 400
URL: https://api.box.com/oauth2/token
Method: POST

in which it is giving:

{
error: "invalid_request",
error_description: "Invalid grant_type parameter or parameter missing"
}

Any insights on what may be breaking?  Thanks!