Ionic 4 Document scanner

I am working on Ionic Document scanner Plugin in Ionic 4.
After the scan, image is saved in Gallery, However i do not get return value from Plugin.
Below is my code
let opts: DocumentScannerOptions = {
quality: 100,
sourceType: srcType,
returnBase64 :true
};
this.documentScanner.scanDoc(opts) .then((res) => {
alert(res); // No alert coming
if (this.useURI) {
this.imageData = (window as any).Ionic.WebView.convertFileSrc(res);
} else { this.imageData = ‘data:image/jpeg;base64,’ + res;
}
}) .catch((error: any) => alert(error));