AngularJS SELECT doesn't validate properly

This is an AngularJS application (1.2.16). I browse to a dialog to edit some item. One of the controls is a multi-SELECT with the following visible values:

incident
work order

These visible values correspond to the following data values:

INCIDENT
WORK_ORDER

This is done through using the ng-options=" ... as ... for ... in ... " pattern, using an enumeration:

var FlexFieldSubjectTypeEnum = {
    INCIDENT:{name:"INCIDENT", description:"incident"},
    WORK_ORDER:{name:"WORK_ORDER", description:"work order"}
}

If have a form pretty much as follows:

<form ng-submit="save(formName)" name="formName" class="form-horizontal">

  ...

    <div class="control-group">
        <label class="control-label">Subject type:</label>
        <div class="controls">
            <select name="subjectType"
                ng-options="type.name as type.description for type in getEnumAsArray('FlexFieldSubjectTypeEnum') | orderBy:'name'"
                ng-model="entity.subjectType"
                required></select>
        </div>
    </div>

Now, if the dialog loads the item ($scope.entity) from the backend and entity.subjectType is set to the first item in the list, the form validation marks it as unset. I have many other dialogs with similar constructs and have not seen this problem anywhere else.

If the item returned from the backend points to the second item (WORK_ORDER), this is nicely represented in the SELECT ("work order") and there is no validation error.

The problem does exist equally when using required or ng-required="true".

The problem does not exist if I remove the required attribute, but then the field also suddenly becomes optional, which is not what I wanted.

Your help much appreciated!