1180: Call to a possibly undefined method... Possibly?

I have buttons, that when clicked will create an instance in one of 3 movie clips within a movie clip.

For instance, this works just fine:

b_IF_4G5_TOP.addEventListener(MouseEvent.CLICK, b_IF_4G5_TOP_add);

function b_IF_4G5_TOP_add(event:MouseEvent):void

{

var b:MovieClip = new IF_4G5_TOP();

b.name = "IF_4G5_TOP";

layout.substrate.addChild(b);

}

but this does not:

p_conn_h.addEventListener(MouseEvent.CLICK, p_conn_h_add);

function p_conn_h_add(event:MouseEvent):void

{

var b:MovieClip = new PC_H(); <- This line produces this error: 1180: Call to a possibly undefined method PC_H.

b.name = "p_conn";

layout.connectors.addChild(b);

}

Which is odd, because as you can see in the first example, I have not "defined" var b:MovieClip = new IF_4G5_TOP();

Both objects exist in the library, are movie clips, both targets and objects are accuratley named and exist.

Am I missing something obvious? Thanks in advance for your help.