diff --git a/lib/zipcelx.js b/lib/zipcelx.js
index d178fb3..b1a1d9a 100644
--- a/lib/zipcelx.js
+++ b/lib/zipcelx.js
@@ -50,7 +50,7 @@ var generateXMLWorksheet = exports.generateXMLWorksheet = function generateXMLWo
 
 exports.default = function (config) {
   if (!(0, _validator2.default)(config)) {
-    return;
+    return undefined;
   }
 
   var zip = new _jszip2.default();
@@ -63,7 +63,7 @@ exports.default = function (config) {
   var worksheet = generateXMLWorksheet(config.sheet.data);
   xl.file('worksheets/sheet1.xml', worksheet);
 
-  zip.generateAsync({ type: 'blob' }).then(function (blob) {
+  return zip.generateAsync({ type: 'blob' }).then(function (blob) {
     _fileSaver2.default.saveAs(blob, config.filename + '.xlsx');
   });
 };
\ No newline at end of file
diff --git a/src/zipcelx.js b/src/zipcelx.js
index ea18823..58dad26 100644
--- a/src/zipcelx.js
+++ b/src/zipcelx.js
@@ -17,7 +17,7 @@ export const generateXMLWorksheet = (rows) => {
 
 export default (config) => {
   if (!validator(config)) {
-    return;
+    return undefined;
   }
 
   const zip = new JSZip();
@@ -30,7 +30,7 @@ export default (config) => {
   const worksheet = generateXMLWorksheet(config.sheet.data);
   xl.file('worksheets/sheet1.xml', worksheet);
 
-  zip.generateAsync({ type: 'blob' })
+  return zip.generateAsync({ type: 'blob' })
     .then((blob) => {
       FileSaver.saveAs(blob, `${config.filename}.xlsx`);
     });