Error: Cannot find control with path: 'specs -> 2' FormArray checkbox list

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I am using reactive forms and in my forms I have an array of string with which I bind my checkbox list.

When I a trying to mark the returned array items to be selected, In console I am able to see the error

.ts code as follow :

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

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

  spec = ['site-1', 'site-2', 'site-4'];

  constructor(private fb: FormBuilder) {}

  ngOnInit() {
    this.form = this.fb.group({
      specs: this.fb.array(['site-1', 'site-2'].map(s => this.fb.control(true)))
    });
  }
}

and html code as follow :

<form [formGroup]="form">
  <div class="col-3">
    <div class="form-group">
      <b>Select Spec Rows</b>
    </div>

    <div class="form-group overflow" formArrayName="specs">
      <div *ngFor="let site of spec; let i = index">
        <input
          type="checkbox"
          [id]="i"
          [formControlName]="i"
          [value]="site"
          (change)="specRowCheck($event,option.id, false)"
        />
        <div [innerHTML]="site"></div>
      </div>
    </div>
  </div>
</form>

ERROR Error: Cannot find control with path: 'specs -> 2'

I checked other questions and its seems I am missing some form group in my html but I am not sure how to apply that