EventSystem.current printing null

Code (CSharp):

1. private void OnEnable()
2. {
3.     Debug.Log(EventSystem.current);
4.     EventSystem.current.SetSelectedGameObject(gameObject);
5. }

Very simple script for selecting a UI element on enable. However, EventSystem.current prints null to the console.

Unity 2019.3.15f1
I have an Rewired event system in the scene.

Also tried putting this in an IEnumerator and waiting a frame before logging to the console - no change.