Adding Validation with Form Alter to the Add To Cart Form

I've tried adding validation with hook_form_alter like so:

$form['#validate'][] = 'quantity_multiples_check_quantity_validate';

I've made sure that it has been added to the add to cart form by doing a print_r on the $form object. It includes this:

    [#validate] => Array
        (
            [0] => commerce_cart_add_to_cart_form_validate
            [1] => my_module_check_quantity_validate
        )

Then I did this, so it should error every time but the form successfully gets submitted.

  function quantity_multiples_check_quantity_validate($form, &$form_state){
    form_set_error('quantity', t('You must specify a valid quantity to add to the cart.'));
  }

Here is all my code that isn't triggering:

 function quantity_multiples_form_alter($form, $form_state, $form_id){
  if(isset($form['#attributes']['class'])){
    if (in_array('commerce-add-to-cart',$form['#attributes']['class'])){
      $form['#validate'][] = 'quantity_multiples_check_quantity_validate';  
    }
  }
 }

  function quantity_multiples_check_quantity_validate($form, &$form_state){
    form_set_error('quantity', t('You must specify a valid quantity to add to the cart.'));
  }