Error in ngModel call

I'm trying a 'Can not read property' reason_social 'of undefined' error when calling a ngModel in my html.

I created an interface like this:

export interface IEmpresas {
  nome_fantasia : string;
  razao_social : string;
  cnpj : string;
}

In my component, I instantiated the interface and created a variable with the type of my interface.

...
private empresa : IEmpresas;
...

and then, I call ngModel in html

<input type="text" [(ngModel)]="empresa.razao_social" id="razao_social">