gulpfile.js:
```javascript
...
browserify({
entries: [ './app/index.js' ],
extensions: [ '.js', '.jsx' ],
debug: true // Add sourcemaps
})
.transform(babelify()) // JSX and ES6 => JS
...
```
A close-up of the issue:
```javascript
.transform(babelify()) // JSX and ES6 => JS
```
The solution:
```javascript
.transform(babelify) // JSX and ES6 => JS
```