No movement on Click & Drag

[AtlasTabula]

Hi, just downloaded this and put it in my project, it seems to be exactly what I'm looking for but I can't for the life of me get the click & drag to work at all.
I have no touch screen so I can't test that.

The issue seems to be with the line:
cam.transform.position -= (cam.ScreenToWorldPoint(deltaPosition) - cam.ScreenToWorldPoint(Vector2.zero));
which only ever returns the camera's default position for me, and I can't seem to get it working.

What exactly am I doing wrong here?