The fix here isn't using the async version, but rather that you're using an empty string when resdoesn't exist (which will result in an empty file). I think we should find out why env.renderString(file.contents.toString(), context) returns undefined sometimes instead.

In: https://github.com/mozilla/nunjucks/blob/master/src/environment.js at line: 501 you can see that nunjucks throws rendering errors if no callback is used with env.renderString. So we must use env.renderString with callback to catch errors within the callback if we can not change nunjucks sourcecode. Its not possible to catch an error which is thrown within another scope. But your right it seems that using the async nunjucks render function is not a good thing here because were dealing with streams. But even if it is not the best solution - my fix fixed the problem in my gulp-task.

If it throws we'll catch it since we're using it inside a try/catch block.