playerprefs, save double?

I'm trying to save double by playerprefs. not sure what I'm wrong. Help!

 public double m_gold = 0.0;
 private double m_goldPerTouch = 1.0;


 private static string DoubleToString(double target){
     return target.ToString ("R");
 }

 private static double StringToDouble(string target){
     if (string.IsNullOrEmpty (target))
         return 0d;
     return double.Parse (target);
 }

 public static void SetDouble(string key, double value){
     PlayerPrefs.SetString (key, DoubleToString (value));
 }

 public static double GetDouble(string key, double defaultValue){
     string defaultVal = DoubleToString(defaultValue);
     return StringToDouble(PlayerPrefs.GetString(key, defaultVal));
 }

 public static double GetDouble(string key){
     return GetDouble (key, 0d);
 }

 void Awake(){
     //PlayerPrefs.DeleteAll ();
     m_gold = GetDouble ("Gold",m_gold);
     m_goldPerTouch = GetDouble ("GoldPerTouch",m_goldPerTouch);
 }

 private static DataController instance;
 public static DataController GetInstance(){
     if (instance == null) {
         instance = FindObjectOfType<DataController> ();
         if (instance == null) {
             GameObject container = new GameObject ("DataController");
             instance = container.AddComponent<DataController> ();
         }
     }

     return instance;
 }

 public void SetGold(double newGold){
     m_gold = newGold;
     SetDouble ("Gold", m_gold);
 }

 public void AddGold(double newGold){
     m_gold += newGold;
     SetGold (m_gold);
 }

 public void SubGold(double newGold){
     m_gold -= newGold;
     SetGold (m_gold);
 }

 public double GetGold(){
     return m_gold;
 }

 public double GetGoldPerTouch(){
     return m_goldPerTouch;
 }

 public void SetGoldPerTouch(double newGoldPerTouch){
     m_goldPerTouch = newGoldPerTouch;
     SetDouble("GoldPerTouch", m_goldPerTouch);
 }

 public void AddGoldPerTouch(double newGoldPerTouch){
     m_goldPerTouch += newGoldPerTouch;
     SetGoldPerTouch (m_goldPerTouch);
 }
     
 public void LoadUpgrade(Boosters boosters){
     string key = boosters.boosterName;

     boosters.level = PlayerPrefs.GetInt (key + "U_Level", 1);
     boosters.gpt = GetDouble("GPC",boosters.gpt);
     boosters.cost = GetDouble("U_Cost",boosters.cost);
 }

 public void SaveUpgrade(Boosters boosters){
     string key = boosters.boosterName;

     PlayerPrefs.SetInt (key + "U_Level", boosters.level);
     SetDouble("GPC", boosters.gpt);
     SetDouble("U_Cost", boosters.cost);
 }

}