E11000 duplicate key error collection in a UNDEFINED value. How avoid this?

I'm making a project with node, mongoose and express.

I have this schema in mongoose

const userSchema = new Schema({
  local: {
    email: {
      type: String,
      lowercase: true,
      unique: true,
      ....
    },
    password: {
      type: String
    }
  },
  google: {
    googleId: String,
    email: String
  },
  ....
};

Because i have two passport strategies (local for the classic email, password and google).

But when i create two accounts with different google accounts. The console shows me this error:

{ MongoError: E11000 duplicate key error collection: xxx.users index: local.email_1 dup key: { : null }

WHAT?? The "Unique" detect duplicated two undefined values?

So. How can i avoid this?

Thank you :)

via nano