TypeInitializationException: Инициализатор типа для vService.CheckService вызвал исключение

Я получаю FaultException от службы WCF следующим образом, когда она вызывается:

2012-04-02 16:26:00.3593|Error|System.ServiceModel.FaultException`1[System.ServiceModel.ExceptionDetail]: The type initializer for 'vService.CheckService' threw an exception. (Fault Detail is equal to An ExceptionDetail, likely created by IncludeExceptionDetailInFaults=true, whose value is:
System.TypeInitializationException: The type initializer for 'vService.CheckService' threw an exception. ----> System.NullReferenceException: Object reference not set to an instance of an object.
   at vService.CheckService..cctor() in d:\working\code\VioletServer\vService\CheckService.cs:line 14
   --- End of inner ExceptionDetail stack trace ---
   at vService.CheckService..ctor()
   at CreatevService.CheckService()
   at System.ServiceModel.Dispatcher.InstanceProvider.GetInstance(InstanceContext instanceContext, Message message)
   at System.ServiceModel.Dispatcher.InstanceBehavior.GetInstance(InstanceContext instanceContext, Message request)
   at System.ServiceModel.InstanceContext.GetServiceInstance(Message message)
   at System.ServiceModel.Dispatcher.InstanceBehavior.EnsureServiceInstance(MessageRpc& rpc)
   at System.ServiceModel.Dispatcher.ImmutableDispatchRuntime.ProcessMessage41(MessageRpc& rpc)
  ...).

Строка 14 CheckService.cs

private static string connStr = ConfigurationManager.ConnectionStrings["violetdb"].ConnectionString;

Что означает исключение в этом контексте и как его разрешить?

//CheckService.cs
public class CheckService : ICheckService
{
    private static string connStr = ConfigurationManager.ConnectionStrings["violetdb"].ConnectionString;
    MessageRepository _repo = new MessageRepository(connStr);

    public CheckService(){}

    public CheckService(MessageRepository repo)
    {
        _repo = repo;
    }

    public void SendMessage(string sender, string recipient, string messagetext)
    {
        _repo.DeliverMessage(sender,recipient,messagetext);
    }
}