Dictionary reference disappearing

Hi guys,

I'm pretty new to C#, and I've run into a bug thats probably an easy and simple fix but I'm just going round in circles now and can't see the solution.

I have a class 'Planet' which contains another class 'Planet_Data' (used for serialization) which contains another class 'Industry'. The Industry class has a Dictionary> member named 'productionSheet' (marked as [System.NonSerializable], constructed like so

 public Industry(Storage s)
 {

     productionUnits = new List<ProductionUnit>();
     ReloadDictionary();
 }

 public void ReloadDictionary()
 {
     productionSheet = new Dictionary<Enums.Commidities, ConsumptionAndProduction>();
     foreach (Enums.Commidities e in Enum.GetValues(typeof(Enums.Commidities)))
     {
         productionSheet.Add(e, new ConsumptionAndProduction(0, 0));
     }
 }

My planet object is constructed like so

 void Awake()
 {
     data = new Planet_data();

     // other members
     data.industry = new Industry(getStore());
     Debug.Log(data.industry.productionSheet);
     // other members
 }

When the scene is launched, the productionSheet is printed out correctly to the console, ie it is instantiated and assigned to at this stage. I then have a function which is called from Update(), which looks like this

 void Turn()
 {
     Debug.Log(data.industry);
     data.industry.ClearProductionSheet(); //
     //

 Industry::
 public void ClearProductionSheet()
 {
     foreach (var e in productionSheet) // null reference exception 
     {
         e.Value.consumption = 0;
         e.Value.production = 0;
     }
 }

The log shows that at the start of turn, the Industry object has been instantiated. However, the Dictionary contained within is now null.

I don't understand why my Dictionary has lost it's reference. Again, this is probably a simple thing I'm missing, but can anyone point out the problem to me?