I am coding a three.js simulation/animation and am having a hard time understanding why hardcoding an array index to a custom class instantiation ("physicsObjects[4]") works, but using the i variable in a for loop ("physicsObjects[i]") does not.

When successful, the updateTail() adjusts the a three.js line position attribute, and then I must set needsUpdate = true as shown in the Three.js documentation to redraw. In the first example, it works successfully (although only for ONE of the objects, 4).

physicsObjects[i] has a .tail property which is a THREE.LineSegments(geometry,material)

    for (let i = 0; i < physicsObjects.length; i++) {
        physicsObjects[4].updateTail()
        physicsObjects[4].tail.geometry.attributes.position.needsUpdate = true
    }

It also works for 2 of my physics objects if I do this.

    for (let i = 0; i < physicsObjects.length; i++) {
        physicsObjects[4].updateTail()
        physicsObjects[4].tail.geometry.attributes.position.needsUpdate = true
        physicsObjects[7].updateTail()
        physicsObjects[7].tail.geometry.attributes.position.needsUpdate = true
    }

But if I want to iterate, it does NOT work. The only difference is that I am indexing with 'i' instead of a hardcoded index?

    for (let i = 0; i < physicsObjects.length; i++) {
        physicsObjects[i].updateTail()
        physicsObjects[i].tail.geometry.attributes.position.needsUpdate = true
    }

This is the error I get.

Uncaught TypeError: Cannot read properties of undefined (reading 'geometry')
    at animate (app.js:159:32)

Here's my entire animate() draw loop for reference:

function animate() {
    //Frame Start up
    requestAnimationFrame(animate);

    //Force Application
    if (frameIndex % 1 == 0) {
        for (let i = 0; i < physicsObjects.length; i++) {
            for (let j = 0; j < physicsObjects.length; j++) {
                if (i !== j) {
                    let f = physicsObjects[i].attract(physicsObjects[j])
                    physicsObjects[i].applyForce(f)
                    physicsObjects[i].updatePhysics()
                    physicsObjects[i].updateGeometry()
     
                }
            }
        }
    }

    //Testing adding a tail to the objects
    for (let i = 0; i < physicsObjects.length; i++) {
        physicsObjects[4].updateTail()
        physicsObjects[4].tail.geometry.attributes.position.needsUpdate = true
    }


    const time = performance.now();
    controls.update(time, prevTime)
    renderer.render(scene, camera);
    stats.update()

    //Frame Shut Down
    prevTime = time;
}

Why can't I iterate with a loop?