ResourceManager.getInstance().getString() returns "undefined" instead of null

Steps to reproduce:
1. Create a resource bundle with key value pairs
2. Load the resource bundle in your application and add them to the resourcemanager instance
3. Use the ResourceManager.getInstance().getString() method with a key that is not in the resource bundle along with a local and put the return value in a string

<s:Label text="{resourceManager.getString('text', 'not-a-key', null, 'en_US')}"/>

Actual Results:
The return value is undefined as a string

Expected Results:
The return (according to the docs) should be null