can`t webview load html page from assets folder

in my app not load html page from assets folder and not error are occur and screen are display as blank

webView = (WebView) findViewById(R.id.htmlviewer);
            webView.getSettings().setJavaScriptEnabled(true);
            webView.loadUrl("file:///android_asset/index.html");
            setContentView(R.layout.htmldesplay);