Does the AngularUI typeahead plugin allow any typed value?

I have an <input> that uses Bootstrap typeahead with Angular UI directive. The display and selection of values in the drop down works fine.

However when I type a value into the input that is not one of the drop down options then this value does not get bound to the model property. Yet what I type remains in the input as if it were valid input.

In the example below, when I type text into the input that is not an option in the autocomplete.referrers array then this text does not get bound to registration.referrer.

Have I configured the input correctly?

<input type="text" 
    name="referrer" 
    ng-model="registration.referrer"
    autocomplete="off"
    typeahead="referrer for referrer in autocomplete.referrers | filter:$viewValue"
    />

Perhaps the restriction on random text input is by design?

Many thanks if you can answer this.