Error: Error calling method on NPObject. #28

I use readAsDataURL to add preview image before crop/upload. I encoutoured "Error: Error calling method on NPObject." with Chrome 24.0 and the same error with FF 18.0.1 said "Error in Actionscript. Use a try/catch block to find error."

This is my code :

YmpCropper.prototype.processPreview = function(files) {
var file, reader;
try {
file = files[0];
this.validateFile(file);
reader = new FileReader();
reader.preview_div = this.preview_div;
reader.img_tag = this.img_tag;
reader.cover_height = this.cover_height;
reader.onload = this.updateCover;
return reader.readAsDataURL(file);
} catch (error) {
return this.displayError(error);
}
};

$('#fileReader').fileReader({
id: 'fileReader',
filereader: 'cropper/filereader.swf',
expressInstall: 'cropper/expressInstall.swf'
});