Issue with FormControl & Rendering

Hi,
I’m new to Ionic 4 and I habe a stupid issue.
I create a page and wants to build a form. The page doesn’t render correctly.
The area with the form is very small and there is a scollbar.
This only happen when i use "formControlName=“firstName”. Without the formcontrol everything is fine.

What is my mistake?

<ion-content>

  <form>
   <ion-item>
      <ion-label>Firstname</ion-label>
      <ion-input type="text" formControlName="firstName"></ion-input>
    </ion-item>
    <ion-item>
        <ion-label>Lastname</ion-label>
        <ion-input type="text" formControlName="lastName"></ion-input>
    </ion-item>
</form>

Open Chrome’s Developer Tools and look in the JavaScript console for errors. If my suspicions are correct, you will see an extremely descriptive message telling you exactly what is wrong and what to do about it. I would put that [formGroup] directive on the form element.