No error when a user has NOT filled out custom details on checkout

There is no error that shows up when a user has not filled in the custom details. This is what I have setup:

<field name = "reference_users" type = "list" label = "Reference" class="required form-control">
<option value = ""> Select One </option>
<option value = "Website"> Website </option>
<option value = "Newsletter"> Newsletter </option>
<option value = "Online Advertisement"> Online Advertisement </option>
<option value = "TV"> TV Ad </option>
</field>

I see an error on other fields like country but not for the custom user details.

Thanks