Can you post the whole function?

And can you look up the line 27 where the error occurs because we can't count the lines if we don't know where to start. There's nothing special you have to take care of. A reference to an object behaves like every other value. Just pass it to the function.

If you can't find the error start debugging. Just use Debug.Log and print all parameters you pass to the function to see if it's null. If it's null you have to go up the chain and find out why it's null. If it's not null you have to go down the chain.

This function is declared inside a class Ai_Controller which inherits from Monobehaviour but is not added to any object as a component.

public static void AiInitHeading( Player_Controller ai, string dir, string deg ){

    ai.QueOrdersHeading.Enqueue( "heading" );

    //load turn direction
    switch( dir ){
        case "to port.":
        case "abeam.":
         ai.QueOrdersHeading.Enqueue( "1.0" );
            break;
        case "to starboard.":
         ai.QueOrdersHeading.Enqueue( "-1.0" );
            break;
        case "galactic.":
         ai.QueOrdersHeading.Enqueue( "0.0" );
            break;
    }

    ai.QueOrdersHeading.Enqueue( deg );

}

It get's called like so from some object's component script:

Ai_Controller.AiInitHeading( myPlayer, stackInput.Pop(), stackInput.Pop() );

Where mPlayer is declared as:

public Player_Controller myPlayer;

With other member variables and initialized in Start() as:

myPlayer = GetComponent< Player_Controller >();

I see two possible reasons:

• GetComponent< Player_Controller >(); returns null because there is no such script attached to this GameObject.
• The QueOrdersHeading variable inside your Player_Controller script has not been initialized.

It turns out when I dragged the Player_Controller component onto the object-- I missed :P But in such a way that I was happily looking at the wrong object going "the component is right THERE, you stupid console!"