Trouble using script to save PSD as PNG

Hi all, 

I have the following code to resize and save a PSD as a PNG. It was working fine, and still works for some psds, but frequently I am getting a user Save a Copy prompt at the doc.saveAs line. Is there something I'm missing? 

var path = File.openDialog().fsName;
    app.rulerUnits = Units.PIXELS;
    var pngOpts = new PNGSaveOptions;
    try {
        var doc = app.open(File(path));
    } catch(e) {
        alert("Could not open psd file " + path + ". " + e);
        return;
    }
    if (doc.width > 1500) {
        doc.resizeImage(1500, null);
    }
    var goodPath = path.replace(/psd|b$/gi,"png");
    doc.saveAs(File(goodPath), pngOpts, true);
    doc.close(SaveOptions.DONOTSAVECHANGES);