Getting "invalid_grant" when validating the authorization code

Hi, everyone,

I'm trying to implement Sign in with Apple and it's been going fine until I tried to valiate the authorization code using the Web Service Endpoint – https://appleid.apple.com/auth/token.

The error I'm getting back from the request is invalid_grant.

I'm sending the following in the POST request to that URL:

code=the_auth_code_given_to_the_app_when_signing_in
client_secret=the_jwt_i_generated
client_id=com.my.app
grant_type=authorization_code

If I decode the client_secret jwt that I created, it looks like this, which is what the documentation says it should look like.

{
    "alg": "ES256",
    "kid": "ABC123DEFG"
}
{
    "iss": "DEF123GHIJ",
    "iat": 1437179036,
    "exp": 1493298100,
    "aud": "https://appleid.apple.com",
    "sub": "com.my.app"
}

Any ideas? Thanks in advance.