Non-Angular Boolean

I have a class in the Angular call user that receives 2 boolean fields

export class UsuariosModel {
    UsuarioId: Number;
    Nome: string;
    Sobrenome: string;
    Email: string;
    Senha: string;
    Corretor: boolean;
    Contrato: boolean;   
}

In my page I have a form using formGroup and all of these fields, being the 'Broker' and 'Contract' of 'type: checkbox'. as follows ...

<form [formGroup]="formUser" novalidate>
    <label>Nome</label>
    <input formControlName="nome" type="text" placeholder="Nome" />

    <label>Sobrenome</label>
    <input formControlName="sobrenome" type="text" placeholder="Sobrenome" />

    <label>Email</label>
    <input formControlName="email" type="text" placeholder="Email" />

    <label>Senha</label>
    <input formControlName="senha" type="text" placeholder="senha" />

    <label>corretor</label>
    <input formControlName="corretor" type="checkbox" />

    <label>Contrato</label>
    <input formControlName="contrato" type="checkbox" />

    <button (click)="inserindoUsuario(formUser.value)">Criar</button>
</form>

In my TypeScript of this page, I have a method that creates an object of type 'User' and receives the parameters of formGroup. But if I do not check the checkbox it comes empty this way 'contract: "' ', and not as false thus' contract: false' ...

Here is my FormGroup and my method ...

FormGroup

constructor(fb: FormBuilder, private api: ChamadaApi, http: Http){
    this.Http = http;

    this.formUser = fb.group({
        nome: new FormControl('', [Validators.required]),
        sobrenome: new FormControl('', [Validators.required]),
        email: new FormControl('', [Validators.required]),
        senha: new FormControl('', [Validators.required]),
        corretor: new FormControl(''),
        contrato: new FormControl('', [Validators.required]),
    });
}

Method

inserindoUsuario(usuarios: any){
    this.usuariosModel = {
         UsuarioId: null,
         Nome: usuarios.nome,
         Sobrenome: usuarios.sobrenome,
         Email: usuarios.email,
         Senha: usuarios.senha,
         Corretor: usuarios.corretor,
         Contrato: usuarios.contrato
     }

     this.api.CreateUser(this.usuariosModel);
}