Structs and null pointer exceptions

I have an IDL looking something like this

module LMS
{

  struct Alarm
  {
    string sensor;
    string zone;
  };

  interface LMSIF
  {
    void raise_alarm(in Alarm aReading);
    void send_alarm_to_rmc(in string alarm);
    string test_case();
  };
};

and a java file looking something like:

class LMSServant extends _LMSIFImplBase
{
   Alarm al;
   //functions here
  public LMSServ()
  {
      //this line causes the problem.
      al.sensor = "Sensor 1";
  }

}

The problem i'm having is when i try to modify or write to the Alarm
struct i get a NullPointerException every time.
Am i doing something wrong?

Do i need to define anything extra to get an IDL struct to work as a
java class?
I tried redefining Alarm as a class with the same variables but this
caused all sorts of other errors..

Any help appreciated :)