About collider() function

Hi, first time using Phaser. My problem is:

When I try to use collider(); function like this,

var my_player;
var grounds;
.
.
.
this.physics.add.collider(my_player, grounds);

It doesn’t work as expected. my_player (in this case a simple image of a ball) just falls down without reacting with the grounds (just a simple rectangle depicting a platform).

Any suggestion or explanation please?
Thanks.

What’s the rest of the code?