Constructors always returning void

This issue seems like it's really simple, but am completely failing to get it to work.

I have a basic data class AI order which has one constructor and one overridden method.

 class AIOrder
 {
     var name:String;
     
     // Constructor
     function AIOrder()
     {
         name = "GenericOrder";
     }
     // Overriden
     function ExecuteOrder(brain:ShipAIBehavior)
     {
     }
 }

I extend the class to represent other order types, overriding the Execute function (I have 10 different types here, this is the simples).

 class AttackNearestOrder extends AIOrder 
 {
     function AttackNearestOrder()
     {
     
         name = "AttackNearest";
     }
     
     function ExecuteOrder(brain:ShipAIBehavior)
     {
         brain.OrderWaitForTarget();
     }
 }

It seems like it should be simple enough. I usually access this by something like this...

 var order:AttackNearestOrder = new AttackNearestOrder();
 aiCraft.AddOrderToOrderList(order );

However, the constructor always returns void. If I try `new AIOrder()` an object of type AIOrder is correctly returned by the constructor, just fails when I try to get the extended class.

The funny thing is that this worked when AIOrder and its extended versions where a subclass of another class (that inherited from MonoBehavior). I moved them out as I needed to be able to construct AIOrders from other classes, and now the trouble has started.

Ideas? Feel like it must be very simple.