dmocks.dynamic(19): Assertion failure for returns(null)

[linkrope]

interface Dependency
{
    string foo();
}

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

    mocker.expect(dependency.foo).returns(null);
    mocker.replay;
    assert(dependency.foo is null);
}

A workaround would be returns(cast(string) null).