Swift 2.0 Course: Structs recap string interpolation failing

During the Structs Recap code test the directions are as follows:

"We learned about instance methods before. 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"

My code is :

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();

When is submit, im being told that my description function isnt returning the right string based on the directions and should use string interpolation. Am I missing something here?