无法验证Angular 6 this.fb.array

import { Component, OnInit } from '@angular/core';
import { FormBuilder, FormGroup, Validators } from '@angular/forms';

@Component({
  selector: 'app-example',
  template: `
    <form [formGroup]="myForm">
      <div formArrayName="myArray">
        <div *ngFor="let control of myForm.get('myArray').controls; let i = index">
          <input [formControlName]="i" />
        </div>
      </div>
      <button (click)="validate()">Validate</button>
    </form>
  `,
})
export class ExampleComponent implements OnInit {
  myForm: FormGroup;

  constructor(private fb: FormBuilder) {}

  ngOnInit() {
    this.myForm = this.fb.group({
      myArray: this.fb.array([
        ['', Validators.required],
        ['', Validators.minLength(3)],
      ]),
    });
  }

  validate() {
    const controls = this.myForm.get('myArray').controls;
    for (let i = 0; i < controls.length; i++) {
      const control = controls[i];
      if (control.invalid) {
        console.log('Validation error:', i, control.errors);
      }
    }
  }
}