Undefined index: add_to_cart in locale_field_entity_form_submit in locale.module on node form submit

I get an error on node form submit (on node save) on my multilingual site.

Notice: Undefined index: add_to_cart in function locale_field_entity_form_submit() (at line 438 in file /site.com/modules/locale/locale.module).

When viewing nodes there are no notices.

What the problem can it be?

/* locale.module */
/* 435 */    foreach (field_info_instances($entity_type, $bundle) as $instance) {
/* 436 */      $field_name = $instance['field_name'];
/* 437 */      $field = field_info_field($field_name);
/* 438 */      $previous_language = $form[$field_name]['#language'];