for each loop exception

hvorfor smider dette loop en :

System.InvalidCastException was unhandled
  Message="Conversion from type 'DBNull' to type 'Double' is not valid."
  Source="Microsoft.VisualBasic"
  StackTrace:
----------------------------------------------------------
Private prisList As ArrayList
  prisList = New ArrayList()


prisList.Add(0.1)
prisList.Add(0.2)
prisList.Add(0.3)
prisList.Add(0.4)

For Each pris As Double In prisList
    Console.Write(pris.ToString() + vbTab)
    counter += 1
Next