instanceOf(Array) assertion fails unexpectedly

[json2d]

Current behavior:

Working off of one of the official examples:
https://github.com/cypress-io/cypress-example-api-testing/blob/master/cypress/integration/example_spec.js#L52-L56

if I replace this section with:

  it('loads 2 items', () => {
    cy.request('/todos')
      .its('body')
      .should('be.instanceOf',Array)
  })

this yields this failure:
AssertionError: expected [ Array(2) ] to be an instance of Array

Desired behavior:

Should pass since it indeed it is an Array

Steps to reproduce: (app code and test code)

1. Clone the project https://github.com/cypress-io/cypress-example-api-testing
2. Replace https://github.com/cypress-io/cypress-example-api-testing/blob/master/cypress/integration/example_spec.js#L52-L56 with snippet above
3. Run npm i, npm start, and npm run cy:open
4. Run example_spec.js test from Cypress ui

Versions

cypress: 3.0.1
os: Windows 10
browser: Chrome 70

[json2d]

According to chai docs:

Due to limitations in ES5, .instanceof may not always work as expected when using a transpiler such as Babel or TypeScript. In particular, it may produce unexpected results when subclassing built-in object such as Array, Error, and Map. See your transpiler’s docs for details:

Would this be the cause?

[json2d]

.should('be.an', 'array') works just fine

would still be nice though to have this mentioned somewhere in docs