Auto Capture Layer Names

I am trying to capture the layer names from an indesign document. The idea behind the script (when finished) is to create a seperate PDF off each layer in the document and naming that document based on the layer name. My script so far can capture the first and last layer name but I cannot get the layers.nextItem to work. I keep getting an Invalid Parameter.

Please can you advise me on what  am doing wrong - Thanks

var myDoc = app.activeDocument;

var myLayers = myDoc.layers.firstItem();

// Box shows first Layer Name

alert (myLayers.name);

for(var i = 0; i < myDoc.layers.length; i++) {

   myLayers = myDoc.layers.lastItem();

// Box shows last layer name but I would like the .lastItem to actually work with .nextItem    

   alert (myLayers.name);

}