Mocking / Spying helper function in Angular Unit tests

I have a helper file with multiple function:

myHelpers.ts

export function fn1(a, b) {
    return a * b;
}

export function fn2(a, b) {
    return a + b;
}

export function fn3(a, b) {
    return a - b;
}

The above file, as well are others are bundled as a module and exported in the index.ts as

import * as Helpers from '/myHelpers';

export { Helpers };

I consume the above module in my app package.json as:

"helper-utils": "file:/local/app/helper"

Which my angular component uses:

import { Helpers } from 'helper-lib';

export class MyComponent {
    constructor() {}

    public btnClick(p1, p2) {
      let value = Helpers.fn1(p1, p2);
      console.log(value);
      return value;
    }
}

The above works fine and if on btnClick() in the GUI i pass 1 and 2, i see the value of 2 in the console.log.

Unit test:

import { ComponentFixture, TestBed, waitForAsync } from '@angular/core/testing';

import { MyComponent } from './myComponent';
import { Helpers } from 'helper-lib';

describe('MyComponent', () => {
  let component: MyComponent;
  let fixture: ComponentFixture<MyComponent>;

  beforeEach(waitForAsync(() => {
    TestBed.configureTestingModule({declarations:[MyComponent]}).compileComponents();
  }));

  beforeEach(() => {
    fixture = TestBed.createComponent(MyComponent);
    component = fixture.componentInstance;
    fixture.detectChanges();
  });

  it('should calc value', () => {
    let mySpy = spyOn(Helpers, 'fn1');

    let value = component.btnClick(1, 2);

    expect(mySpy).toHaveBeenCalled();
  });
});

Error:

Error: <spyOn> : fn1 is not declared writable or has no setter

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