Why this script unfunction?

Hi, everyone

I add an  UndoModes.FAST_ENTIRE_SCRIPT to the script, but unfunction, can you fix it?

app.doScript('main()', ScriptLanguage.JAVASCRIPT, undefined, UndoModes.FAST_ENTIRE_SCRIPT, "Indent Paragraphs"); 

function main(){

var curSel = app.selection[0]; 
for ( var i = 1; i < app.documents.length; i++) { 
    var nextDoc = app.documents; 
    if (!curSel) { alert( "Nothing selected!" ); exit(); } 
    curSel.duplicate( nextDoc.masterSpreads[0] ); 
}
}

John