Group in FormGroup

I am new to ionic, i will create a form with formControlName input and formgroup in form tag, and in .ts file , i code as below, but after many search and try getting only one error : TypeError: Cannot read property ‘group’ of undefined. Please help me to solve this issue, I was search many examples and forms about form but in all that of group is define and they not get any error. Thanks in advance…

constructor(public navCtrl: NavController, public restapiService: ReastApiServiceProvider, public alertController: AlertController, public formBuilder: FormBuilder) {
this.getUsers();

this.myForm = this.formBuilder.group({title: [Validators.required, Validators.minLength(5)],
                                       description: ['']});

this.myForm.valueChanges.subscribe(data =>this.myFormOnDataChange(data));