Ion-segment converts boolean to string ionic-v4

I’m using ion-segment with ngModel. The variable is cast as a boolean, but ion-segment keeps getting away with converting it to a string. Here’s my example code:

            <ion-segment [(ngModel)]="symptom.symptomValue" color="primary">
                <ion-segment-button [value]="true">
                <ion-segment-button [value]="false">

This is something that was working fine in Ionic 3 but is failing as I’m migrating to Ionic 4.