Collider between two groups doesn't fire

[Allan11]

I have a function which is called on update

import enemy from './enemy';

export const shoot = ({ scene }) => {
  scene.input.on('pointerdown', function(pointer) {
    if (pointer.button === 0) {
      gun({
        scene,
        fromX: playerObj.x,
        fromY: playerObj.y,
        toX: pointer.position.x,
        toY: pointer.position.y,
        enemy,
      });
    }
  });
};

In my gun function

const gun = ({ scene, fromX, fromY, toX, toY, enemy }) => {
  gunPhysicsGroup = scene.physics.add.group();

  bullet = scene.add.circle(fromX, fromY, 2, 0xff0000);

  gunPhysicsGroup.add(bullet);

  const delta = Math.sqrt(Math.pow(toX - fromX, 2) + Math.pow(toY - fromY, 2));
  const velocityX = (speed / delta) * (toX - fromX);
  const velocityY = (speed / delta) * (toY - fromY);

  gunPhysicsGroup.setVelocity(velocityX, velocityY);

  //The callback here doesn't fire
  scene.physics.add.collider(bullet, enemy, () => console.log('heeee'));
};

and an enemy is this function

let enemyPhysicsGroup = null;
let enemyObj = null;

const enemy = ({ scene }) => {
  enemyPhysicsGroup = scene.add.group({
    bounceX: 1,
    bounceY: 0.2,
  });

  for (var i = 1; i < 10; i++) {
    enemyObj = scene.add.circle(300 + i * 20, 100, 10, 0xabcdef);
    enemyObj = scene.add.circle(300 + i * 20, 120, 10, 0xabefac);
    enemyObj = scene.add.circle(300 + i * 20, 140, 10, 0xeba2bc);
  }
};

export default enemy;

Everything works fine - the bullet gets fired on mouse click, the enemy is drawn but the collision between them is not registered.

Any help will be appreciated.