Namespace errors when importing Photon Voice

I have an existing PUN+ project I want to integrate Photon Voice into. However, when I import the Photon Voice asset I get the following 4 errors:

"Assets/Photon Unity Networking/Demos/PunBasics-Tutorial/Scripts/GameManager.cs(25,43): error CS0234: The type or namespace name `MonoBehaviour' does not exist in the namespace `ExitGames.Photon'. Are you missing an assembly reference?

Assets/Photon Unity Networking/Demos/PunBasics-Tutorial/Scripts/Launcher.cs(25,40): error CS0234: The type or namespace name `PunBehaviour' does not exist in the namespace `ExitGames.Photon'. Are you missing an assembly reference?

Assets/Photon Unity Networking/Demos/PunBasics-Tutorial/Scripts/PlayerAnimatorManager.cs(17,53): error CS0234: The type or namespace name `MonoBehaviour' does not exist in the namespace `ExitGames.Photon'. Are you missing an assembly reference?

Assets/Photon Unity Networking/Demos/PunBasics-Tutorial/Scripts/PlayerManager.cs(22,45): error CS0234: The type or namespace name `PunBehaviour' does not exist in the namespace `ExitGames.Photon'. Are you missing an assembly reference?"

I tried creating a blank new project, importing PUN+ and then Photon Voice but received the same errors so it doesn't appear to be an issue with my project.

I'm on Unity 5.3.5p6. I received the same issue on p4.