角度材料md-选择空值验证不起作用

<mat-form-field>
  <mat-label>选择项</mat-label>
  <mat-select required [(ngModel)]="selectedOption">
    <mat-option value="option1">选项1</mat-option>
    <mat-option value="option2">选项2</mat-option>
    <mat-option value="option3">选项3</mat-option>
  </mat-select>
  <mat-error *ngIf="selectedOption.invalid">请选择一个选项</mat-error>
</mat-form-field>

import { Component } from '@angular/core';
import { FormBuilder, Validators } from '@angular/forms';
import { MatSelectModule } from '@angular/material/select';

@Component({
  selector: 'app-my-form',
  templateUrl: './my-form.component.html',
  styleUrls: ['./my-form.component.css']
})
export class MyFormComponent {
  selectedOption: FormControl;

  constructor(private formBuilder: FormBuilder) {
    this.selectedOption = this.formBuilder.control('', Validators.required);
  }
}