TypeError: Cannot read property 'length' of null

[jeffsaremi]

Following one of your examples. I want to wrap the contents of a file in some additional lines:

gulp.task('build', function () {
	return gulp.src(['./lib/**'])
		.pipe(tap(function(file) {
			file.contents = Buffer.concat([
				new Buffer('modules.define("' + file.path + '", function(module, require) {\n'),
				file.contents,
				new Buffer("\n});\n")
			]);
		}))
		.pipe(concat('index.js'))
		.pipe(header(preamble))
		.pipe(footer(closing))
		.pipe(babel())
		.pipe(gulp.dest('dist'));
});

Running gulp produces:

D:\repos\project>gulp
[09:09:59] Using gulpfile D:\repos\project\gulpfile.js
[09:09:59] Starting 'build'...
buffer.js:467
      length += list[i].length;
                        ^

TypeError: Cannot read property 'length' of null
    at Function.concat (buffer.js:467:25)
    at D:\repos\project\gulpfile.js:61:27
    at DestroyableTransform.modifyFile [as _transform] (D:\repos\project\node_modules\gulp-tap\lib\tap.js:53:11)
    at DestroyableTransform.Transform._read (D:\repos\project\node_modules\gulp-tap\node_modules\through2\node_modules\readable-stream\lib\_stream_transform.js:184:10)
    at DestroyableTransform.Transform._write (D:\repos\project\node_modules\gulp-tap\node_modules\through2\node_modules\readable-stream\lib\_stream_transform.js:172:83)
    at doWrite (D:\repos\project\node_modules\gulp-tap\node_modules\through2\node_modules\readable-stream\lib\_stream_writable.js:428:64)
    at writeOrBuffer (D:\repos\project\node_modules\gulp-tap\node_modules\through2\node_modules\readable-stream\lib\_stream_writable.js:417:5)
    at DestroyableTransform.Writable.write (D:\repos\project\node_modules\gulp-tap\node_modules\through2\node_modules\readable-stream\lib\_stream_writable.js:334:11)
    at write (D:\repos\project\node_modules\gulp\node_modules\vinyl-fs\node_modules\through2\node_modules\readable-stream\lib\_stream_readable.js:623:24)
    at flow (D:\repos\project\node_modules\gulp\node_modules\vinyl-fs\node_modules\through2\node_modules\readable-stream\lib\_stream_readable.js:632:7)