Form_set_error for array of fields I got below code in form_validation

$form_state['values']['field_address'][LANGUAGE_NONE][0][postal_code];


I've tried below tricks but it didn't make red color in textbox.

form_set_error('field_address][postal_code', 'error');
form_set_error('field_address[und][0][postal_code', 'error');
form_set_error("field_address[und][0]['postal_code']", 'error');
form_set_error('field_address[LANGUAGE_NONE][0][postal_code', 'error');
Your form_set_error like that

form_set_error('field_address][postal_code', t('enter postal code'.));