No best common type exists among return expressions

When I use Collection2 in angular2-meteor project, these kinds of codes from demo always give me warning in the terminal:

No best common type exists among return expressions.

How can I improve the codes? Thanks

{
  createdAt: {
    type: Date,
    autoValue: function() {
      if (this.isInsert) {
        return new Date();
      } else if (this.isUpsert) {
        return {$setOnInsert: new Date()};
      } else {
        this.unset();
      }
    }
  }
}