Am I Doing String Interpolation Wrong?

For this code challenge, I am supposed to be creating an initializer method for the struct "RGBColor", assigning values to the first 4 properties, and then creating a string (through interpolation) that lists out the values for the first 4 properties. I have this running in Xcode, and it seems to be working exactly as I'd expect it should be. When I call RGBColor, I pass in some doubles, and the description reads correctly.

The lesson is telling me to make sure I am using string interpolation to get the correct string for my description value. It seems to me like everything checks out, and I've been looking this up and down for any minor errors I may have missed for about half of an hour. What have I done wrong?

struct RGBColor {
  let red: Double
  let green: Double
  let blue: Double
  let alpha: Double

  let description: String

  // Add your code below

  init(red: Double, green: Double, blue: Double: alpha: Double) {
    self.red = red
    self.green = green
    self.blue = blue
    self.alpha = alpha

    description = "red: \(red), green: \(green), blue: \(blue), alpha: \(alpha)"
  }
}