Build a Simple iPhone App w/ Swift code challenge

The challenge is pretty simple, yet I can't for the life of me get things to work. Instead, I keep getting the following error:

Bummer! Make sure the implementation of description matches the directions. Hint: Use string interpolation!

I'm going bonkers, because I am using string interpolation and am pretty sure it's exactly what the instructions said it should be. ?

These are the instructions: For this task, add an instance method to Post.

Name the method description. It takes no parameters and returns a string that describes the post instance. For example given a title: "iOS Development", author: "Apple", and a tag named "swift", the description would read as follows"

"iOSDevelopment by Apple. Filed under swift"

Once you have an instance method, call it on the firstPost instance and assign the result to a constant named postDescription

struct Tag {
  let name: String
}

struct Post {
  let title: String
  let author: String
  let 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()