How to outline an ng select box red via validation

I have a few fields in a form which need to be not null. I have enforced validation using the angular validator class : Validators.required

This seems to work well for text input fields but not ng select fields.
The validation holds up fine, it won’t let you submit the form. However, unlike the text input fields, the ng select field does not highlight its text and its line in red to signify to the user that is the field with the issue.

Is there a better way to enforce this color property? I also tried the required tag in the HTML portion, same results.

Source: Angular Questions