How to add <ion-radio-group> to Reactive Form in Ionic / Angular app?

I am trying to add an <ion-radio-group> containing 2 <ion-radio> to a reactive form in my Ionic / Angular app.

Below I am initialising the form, & creating a method executed when I submit the form:

ngOnInit() {
    this.form = new FormGroup({
      userType: new FormControl(null, {
        updateOn: 'blur',
        validators: [Validators.required]
      }),
}

onSubmit() {
    if (this.form.invalid) {
      console.log('form invalid');
      return;
}

Here I am giving an <ion-radio-group> a formControlName :

<form [formGroup]="form">
    <ion-radio-group formControlName="userType">
        <ion-row>
                <ion-col size-sm="3" offset-sm="3">
                    <ion-label>User Type:</ion-label>
                </ion-col>
        </ion-row>
        <ion-row>
            <ion-col size-sm="3" offset-sm="3">
                <ion-item lines="none">
                    <ion-label>Customer</ion-label>
                    <ion-radio value="customer"></ion-radio>
                </ion-item>
            </ion-col>
            <ion-col size-sm="3">
                <ion-item lines="none">
                    <ion-label>Mechanic</ion-label>
                    <ion-radio value="mechanic"></ion-radio>
                </ion-item>
            </ion-col>
        </ion-row>
    </ion-radio-group>
    <ion-button color="primary" (click)="onSubmit()">Submit</ion-button>
</form>

When I execute onSubmit() , form invalid is logged to the console.

Can someone please tell me how I can resolve this error?

Also - I tried the below change, but now I get this error message when the page loads:

No value accessor for form control with name: ‘userType’

<ion-row radio-group formControlName="userType">
    <ion-item>
      User Type
    </ion-item>
    <ion-col>
       <ion-item>
        <ion-label>Customer</ion-label>
        <ion-radio value="customer"></ion-radio>
      </ion-item>
    </ion-col>
    <ion-col>
       <ion-item>
        <ion-label>Mechanic </ion-label>
           <ion-radio value="mechanic"></ion-radio>
           </ion-item>
        </ion-col>
</ion-row>