Not stubbing my method, instead it calls it

Test:

describe('Event Widget', () => {
  it('should throw error when no option are passed in', () => {
    sinon.stub(ErrorHandler, 'renderErrorTemplate');

    ErrorHandler('divId');
    ErrorHandler.renderErrorTemplate.should.have.been.called;
  });
});

Code:

const ErrorHandler = require('../../helpers/error_handler');

(() => {
  let events = (divId, options, env) => {
    if (isUndefined(options)) {
      ErrorHandler.renderErrorTemplate(divId, 'Options cannot be undefined.');
    }

  ...
});

Expected result: Test should pass, method should be mocked. Actual method should never be called, instead use mock.

Actual Result: It calls the real method and tries to render a marco template. As a result it explodes on "unknown characters such as <

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