script leaves FrameMaker file locked

The following script updates the specified RoboHelp project with any changes to the FrameMaker source. It works fine, except it leaves the last .fm file in the book locked (i.e. there's an .lck file in the FrameMaker source dircectory). How can I fix that? Or is this a bug?

var projectPath = "<path>\\<project>.xpj";

main();

function main()

{

        if(projectPath == "")

        {

               //Error!. Quit RoboHelp

               alert("Project path is not defined. \nPlease update the 'projectPath' variable in the script.");

               RoboHelp.quit();

        }

        RoboHelp.openProject(projectPath); // open the project

        RoboHelp.project.updateAll(true); // force-update FrameMaker source

        RoboHelp.closeProject(); // close the project

        RoboHelp.quit(); //quit RoboHelp

}