Solvedangular Property 'controls' does not exist on type 'AbstractControl'

[x] bug report

ERROR in ng:///path/to/app/my/custom.component.html (7,10): Property 'controls' does not exist on type 'AbstractControl'.

this.inputForm = this._builder.group({
  base: this._builder.group({
    monthly: [
      '', Validators.compose([Validators.required, Validators.pattern(regInt)])
    ],
   ...
  }),
  user: this._builder.group({
    ...
  })
});

<div class="form-group row" [ngClass]="{'has-danger': inputForm.controls.base.controls.monthly.dirty && !inputForm.controls.base.controls.monthly.valid}">
  <label class="col-4 col-form-label" for="inp-monthly">
    Monthly Expenses
  </label>
  <div class="col-8">
    <div class="input-group">
      <span class="input-group-addon" id="inp-monthly-s">$</span>
      <input class="form-control" id="inp-monthly" formControlName="monthly" aria-describedby="inp-monthly-s"
        type="number" min="1" placeholder="Add up all your costs (rent, salary, telephone, internet, inventory costs, etc)" />
    </div>
    <div class="form-control-feedback" [hidden]="inputForm.controls.base.controls.monthly.pristine || inputForm.controls.base.controls.monthly.valid">
     Please enter a quantity.
    </div>
  </div>
</div>

buildForm() {
    let fb = this.formBuilder;

    this.form = fb.group({
        animals: fb.group({
            cat: '',
            dog: '',
            other: ''
        })
    });
}

get animals(): FormGroup {
    return this.form.get('animals') as FormGroup;
}

<div class="button-list" formGroupName="animals">
    <label class="button" *ngFor="let animal of animals.controls | keys" [ngClass]="{'active': form.value.animals[animal]}">
        <input type="checkbox" formControlName="{{animal}}">
        <span class="button__text">{{animal}}</span>
    </label>
</div>

formGroup: FormGroup;
formArray: FormArray;

initForm() {
  this.formArray = new FormArray([]);

  //init form array here - push FormGroups

 //then insert form array
 this.formBuilder.group({
 forms: formArray;
 });
}