Script for Applying master Spread

Hi,

This Script is for Applying CO-Opener master page  wherever CN is applied, When we run the script, it alerts Sucess.But master spread is not applied . Please resolve.

var mydoc=app.activeDocument;
var mypara = mydoc.stories.item(0).paragraphs.everyItem().getElements();
for(i=0; i<mypara.length; i++){    if (myPara[i].appliedParagraphStyle.name=="CN"){           mypara[i].parentTextFrames[0].parent.appliedMaster = mydoc.masterSpreads.item("CO-Opener");        }    }
alert("Master Page applied Sucessfully")