Weird Error Implementing GraphQL Go Resolvers

Here is my resolver:

// CreateBook is the resolver for the createBook field.
func (r *mutationResolver) CreateBook(ctx context.Context, input model.NewBook) (*model.Book, error) {
book := &model.Book{
    ID:              input.ID,
    Default_user_id: input.DefaultUserID,
}

n := len(r.Books)
if n == 0 {
    r.Books = make(map[string]*model.Book)
}

r.Books[input.ID] = book
return r.Books[input.ID], nil
//panic(fmt.Errorf("not implemented: Teachers - teachers"))
}

Here's my Books declaration in my Resolver struct:

Books map[string]*model.Book

Here's my response when I call the mutation:

{
  `"errors": [
    {
      "message": "internal system error",
      "path": [
        "createBook",
        "id"
      ]
    }
  ],
  "data": {
    "createBook": null
  }
}

I've attempted the same implementation with an array and was able to successfully query and mutate, but when I use a map to represent the books I receive the above error.

Please help I'm so lost. See above for my code and the mutation I attempted.