createCustomToken generates invalid Token about dart_firebase_admin HOT 5 OPEN

Of course so my workflow is:

1. HTTP Request to Dart Backend (DartFrog) to get the token

2. On the Server Endpoint I have the following code to generate the token
   auth.createCustomToken(uid, developerClaims: {'role': 'm1_admin'});

3. On the client I use the token I genereated in step 2 to sign in with the flutter client sdk:
   FirebaseAuth.instance.signInWithCustomToken(token);

This is where I get the error:
error: [firebase_auth/invalid-custom-token] The custom token format is incorrect. Please check the documentation.

from dart_firebase_admin.

I'm also having this issue, I'm just using the createCustomToken without any claims and I can't use the token with the signInWithCustomToken endpoint. Same error about the formatting

from dart_firebase_admin.

When i send the token back to the client and trie the LoginWithToken method I get:

What does that mean exactly?
Maybe some code samples would help here

from dart_firebase_admin.