How to match collection contents in Spock interactions?

Given I have the following method in a class that I mock:

class Foo {
   public void doSomething(Collection<String> input) {
     //...
   }
}

Now I mock this class in my Spock test and I want to verify an interaction:

def test() {
    setup:
    def myMock = Mock(Foo)

    when:
    def hashSet = new HashSet<String>(['foo', 'bar'])
    myMock.doSomething(hashSet)

    then:
    1 * myMock.doSomething(['foo', 'bar'])

}

This interaction however doesn't trigger. What is really strange is that the output is telling me:

too few invocations for:

1 * myMock.doSomething(['foo', 'bar'])   (0 invocations)

Unmatched invocations (ordered by similarity):

1 * myMock.doSomething(['foo', 'bar'])

So it basically told me that there was no invocation that looked like the one I was expecting but there was another one which ... ermm looked like the one I was expecting.

Am I doing something wrong here or is this a limitation of Spock and I need to check the collection contents in a closure like

1 * mock.doSomething( { it == ['foo', 'bar'] })