Oops!  Mistake on my part ahead!  

In the next video, I tell you to run the command npm install --save react-router@4.0.0 .

The React Router project has now been broken up into a couple of different libraries.  The correct command to run is npm install --save react-router-dom@4.2.2 <--run this instead.  This library contains the flavor of react-router related to working with the DOM, as opposed to other platforms (like React Native).