How can I batch turn on/turn off layers in a large group of files

I lifted this code from another discussion that purports to say it works, but it doesn't.

I keep getting the Error: Object does not support the property or method 'length'

I've looked at other batch process scripts that that line of code seems to be correct.

I know this is a very simple script, and that the turn on/turn off layer part of the script works perfectly fine (on individuall pages), but for some reason I can't get it to batch process.

I have a multi-version set of files with layers such as "de" for germany, "us" for United States, etc. that I need to batch turn on so that I can print.

Here is what I have:

var myFolder = Folder.selectDialog("Select Indesign Folder");

var myIndsnFiles = myFolder.getFiles("*.indd");

for(k=0; k<myIndsnFiles.length; k++)

{

   var myIndsnFiles = app.open(myIndsnFiles[k]);

var myIndsnFiles = app.documents[0];

var answersLayer = myIndsnFiles.layers.item("us");

answersLayer.visible = false;

    app.activeDocument.close(SaveOptions.yes);

    }

Can you fix/explain what I'm doing wrong?