If the key does not exist, make sure to throw the InvalidKey error

I get the above error and nothing in the output file when I use the following code in "Error Handling in Swift/Handling Errors/Task 1 of 3":

guard data != nil else { throw ParserError.emptyDictionary } guard let exists = data?.keys.contains("someKey") else { throw ParserError.invalidKey } guard let keyVal = data?["someKey"] else { throw ParserError.emptyDictionary }

I'm not sure what I'm doing wrong here. Also, is there a better way to write this given that the compiler is telling me that I don't use the 'exists' and 'keyVal' constants?

enum ParserError: Error {
  case emptyDictionary
  case invalidKey
}

struct Parser {
  var data: [String : String?]?

  func parse() throws {
        guard data != nil else {
            throw ParserError.emptyDictionary
        }
        guard let exists = data?.keys.contains("someKey") else {
            throw ParserError.invalidKey
        }
        guard let keyVal = data?["someKey"] else {
            throw ParserError.emptyDictionary
        }
  }
}

let data: [String : String?]? = ["someKey": nil]
let parser = Parser(data: data)