'Error: Cannot find control with path: 'linkedUrns -> [object Object]'

I'm trying to create a widget which allows the user to enter Unique Reference Numbers (strings)

Here is the template for the component:

<div class="card foregroundlight backgrounddark" >
    <div class="card-header">
        <div class="d-flex align-items-center">
            <div class="mr-auto">
                <h4>Linked URNs</h4>
            </div>
            <div class="d-flex align-items-center">
                <button type="button" class="btn btn-secondary" (click)="addFormControl()">Add Linked URN</button>
                <lib-tooltip-button fieldName="linkedUrns"></lib-tooltip-button>
            </div>
        </div>
    </div>
    <div *ngIf="linkedUrnsArray.controls?.length > 0" class="card-body" [formGroup]="formGroup">
        <div formArrayName="linkedUrns">
            <div [formControlName]="linkedUrn" *ngFor="let linkedUrn of linkedUrnsArray.controls; index as i">
                <div class="row">
                    <div class="col-md-5">
                        <h5>URN</h5>
                        <div class="form-group">
                            <input class="form-control" formControlName="linkedUrn" placeholder="Enter URN">
                        </div>
                    </div>
                    <div>
                        <button class="button btn-danger" (click)="removeFormControl(i)"  aria-label="delete">
                            <mat-icon>delete</mat-icon>
                        </button>
                    </div>
                </div>
            </div>
        </div>
    </div>
</div>

And the ts file:

import { Component, OnInit, Input } from '@angular/core';
import { FormGroup, FormBuilder, FormArray, FormControl } from '@angular/forms';
import { ValidatorService } from '../../services/validator.service';

@Component({
  selector: 'lib-app-linked-urns-widget',
  templateUrl: './linked-urns-widget.component.html',
  styleUrls: ['./linked-urns-widget.component.css']
})
export class LinkedUrnsWidgetComponent implements OnInit {

  @Input() formGroup: FormGroup;
  linkedUrnsArray: FormArray;

  constructor(private formBuilder: FormBuilder/* ,  private validatorService: ValidatorService */) {
  }

  ngOnInit() {
    this.linkedUrnsArray = this.formGroup.get('linkedUrns') as FormArray;
  }

  addFormControl() {
    this.linkedUrnsArray.push(this.createNewLinkedUrn());
    console.log('!!this.linkedUrnsArray', this.linkedUrnsArray);
  }

  createNewLinkedUrn(): FormControl {
    return this.formBuilder.control(
      ''
    /* ,this.validatorService.linkedUrnValidator */);
  }

  removeFormControl(i) {
    this.linkedUrnsArray.removeAt(i);
  }
}

You can see that the LinkedUrnsWidgetComponent takes an input of formGroup from it's parent - and I've logged that formGroup to the console so you can see that the linkedUrn formControl exists within it.