How to handle certain contracts?

[linkrope]

interface Dependency
{
    int foo()
    out(result) { assert(result != int.init); }
}

unittest
{
    auto mocker = new Mocker;
    auto dependency = mocker.mock!Dependency;

    mocker.expect(dependency.foo).returns(42);
    mocker.replay;
    assert(dependency.foo == 42);
}

fails in dmocks.mocks.ExpectationSetup dmocks.mocks.Mocker.expect: Assertion failure