"are you missing a using directive or an assembly reference?"

No doubt this is probably an easy user error, but while builds succeed locally, I'm getting errors like this in GitLab CI:

 Assets/Grid/GameGrid.cs(9,19): error CS0234: The type or namespace name 'InputSystem' does not exist in the namespace 'UnityEngine' (are you missing an assembly reference?)

This is for the new (currently preview) Unity Input System, which I had to install via the Package Manager.

So...

1. Ideas on what the fix is here?
2. If this is likely a common problem, can we document it in the readme? 😄