iOS Build a Simple iPhone App with Swift Getting Started with iOS Development Swift Recap Part 1

Structs.swift challenge

This code runs per my interpretation of the directions in min playground, but I keep getting an error here saying to make sure I am using string interpolation. What am I missing here?

structs.swift
struct Tag {
  let name: String
}

struct Post {
    var title: String
    var author: String
    var tag: Tag

  func description() -> String {
     return "\(title) by \(author). Filed under \(tag)"
  }
}

let firstPost = Post(title: "iOS Development", author: "Apple", tag: Tag(name: "swift"))
let postDescription = firstPost.description()