data-api-builder: Mutations are not working with Apollo Client : passing variables

I am using an Angular Client with angular-apollo https://apollo-angular.com/docs/data/mutations and Mutations are throwing an error even though the variables are passed.

This is with Cosmos DB as storage.

Client Code :

const post_SaveHero = gql`
mutation ($id : String!, $publisher : String!, $superhero :String!, $characters:String!){
  addHero(id : $id , publisher : $publisher , superhero : $superhero , characters: $characters )
 {
  superhero
}
}`;

Invoking it as,

  updateHero(hero: Hero){
    console.log("$$$HERO",hero.id);
    console.log("$$$HERO OBJECT",JSON.stringify(hero));
    return this.apollo.mutate({
      mutation: post_SaveHero,
      variables: {
        id: hero.superhero,
        publisher: hero.publisher,
        superhero: hero.superhero,
        characters: hero.characters
      }
    }
    );
  }

and the error thrown as,