Strange null reference problem within enum?

Hi All,

We've been having a strange problem. Recently we have seen null pointer exception coming out from our code. The exception trace is dead simple:

Message: The run has errored with description: Object reference not set to an instance of an object.
Object reference not set to an instance of an object.
-------------------------
User:
Source: mscorlib

Server stack trace:
at System.Collections.Hashtable.get_Item(Object key)
at WGP.Adapter.IntToPrataEnum(String prataCode, String val)
at WGP.Adapter.Curve.BuildCurves(Double[] date, Int32[] instrument, String[] currencies)
...

Anyway what really strange is that the code in IntToPrataEnum method is dead simple and straight forward. As far as I can see, I couldn't find a single problem here. Code is modified, but generally follows the same logic. 2 strings, switches and depending on the content return the correct enum cast into integer.

public static int IntToPrataEnum(string prataCode, string val)
{
  switch(prataCode)
  {
    case "Interpolation":
      switch(val)
      {
        case "LINEAR":
          return (int)WGP.InterpMethod.Linear;
        case "LOG_LINEAR":
          return (int)WGP.InterpMethod.LogLinear;
        case "SPLINE":
          return (int)WGP.InterpMethod.Cubic;
        default:
          throw new ArgumentException("Interpolation error");
      }
    case "Day Count":
      switch(val)
      {
        case "ACT/360":
        case "ACT_360":
          return (int)WGP.DayCount.Actual360;
        case "ACT_365":
        case "ACT/365":
          return (int)WGP.DayCount.Actual365Fixed;
        case "ACT_ACT":
        case "ACT/ACT":
          return (int)WGP.DayCount.ActualActual;
        default:
          throw new ArgumentException("Day count error");
      }
    default:
      throw new ArgumentException("Unrecognised parameter");
  }
}

Now the system is multi-threaded, and we're thinking there's something in .NET that makes initialisation and access of enum can throw a null pointer exception

The fact that it was thrown from a hashtable is odd, there's no reference of Hashtable in the method in question and I can only assume that's how enum is implemented internally within .NET

Any ideas

Cheers