Why doesn't my chai rejectedWith assertion work?

I'm using Coffeescript, Mocha, and chai, and I have the following code:

class Herp extends Error
message = "derpy!"

class Foo
    @bar: ->
        new Promise(() =>
            throw Herp(message)
        )

describe 'Berp', ->
    it 'should herp a derp', ->
        Foo.bar().should.eventually.be.rejectedWith(Herp(message))

I do not understand why, when I run this test, it tells me:

AssertionError: expected promise to be rejected with [Error: derpy!] but it was rejected with [Error: derpy!]

Cause that message looks an awful lot like it's saying that it expected it to be rejected with the exact thing it was rejected with. What am I doing wrong here?

❯ grep chai package.json         
    "chai": "^3.5.0",
    "chai-as-promised": "^5.3.0",

❯ node --version
v6.4.0

❯ npm --version 
3.10.3