OnCollisionEnter2D not called

void OnCollisionEnter2d(Collision2D other){

I have this method on both objects. The Rigidbody stops the character from going through the other collider, but the debug never gets called. I tried Discrete and Continuous Collision Detection; neither made a difference. I’m using Unity 5. Am I missing something here, or is this a bug with Unity?

Capitalisation matters…