What causes the failure to validate nested form fields in this Angular 14 app?

I have been developing an e-commerce app with Angular 14 and Angular Material.

I run into a problem while tiring to display specific (custom) validation messages for some of the fields of the registration form.

In the form component's Typescript file, I have:

import { FormService } from '../../services/form.service';

export class FormComponent implements OnInit {

  constructor (
        private formService: FormService
      ) { }
      

      public form: FormGroup = new FormGroup({
        firstName: new FormControl('', Validators.required),
        lastName: new FormControl('', Validators.required),
        email: new FormControl('', [Validators.required, Validators.email]),
        
        workInfo: new FormGroup({
            companyName: new FormControl('', Validators.required),
            companyCity: new FormControl('', Validators.required),
        }),
      });

      ngOnInit(): void {
      }
  
}

In the template:

<form [formGroup]="form" novalidate>
    <mat-form-field appearance="outline" floatLabel="always">
        <mat-label>First name:</mat-label>
        <input matInput formControlName="firstName">
        <mat-error *ngIf="form.controls['firstName'].errors?.['required']">What's your first name?</mat-error>
    </mat-form-field>
    
    <div formGroupName="workInfo">
        <mat-form-field appearance="outline" floatLabel="always">
        <mat-label>Company name:</mat-label>
        <input matInput formControlName="companyName">
        <mat-error *ngIf="form.controls['companyName'].errors?.['required']">What company do you work for?</mat-error>
    </mat-form-field>
    </div>
</form> 

Using this syntax also does not work:

<mat-error *ngIf="form.controls['workInfo.companyName'].errors?.['required']">What company do you work for?</mat-error>

The problem

Although I can display specific validation messages for the firstName field (or any other that isn't enclosed in a form group), I can not do the same for any of the fields inside the <div formGroupName="workInfo"> element.

I get an TypeError: Cannot read properties of undefined (reading 'errors') for any pf them.

Questions

1. What am I doing wrong?
2. What is the easiest and most reliable way to achieve the desired result?