I have a test where I create a mock Examine search result and test some code which should run if there are any results

line of code I need to test:

if (results.Any())
{
    ...do stuff
}

And in the test:

private Mock<ISearchResults> _mockSearchResults;

in the setup:

_mockSearchResults = new Mock<ISearchResults>();

At this point the mock results do not contain anything so if statement won't be true, I'm guessing I need to add a search result item to the results but I'm not sure how to, or if that's even correct. Here's what I was thinking, but it gives the error 'Invalid setup on an extension method' :

_mockSearchResults.Setup(x => x.Append(_mockSearchResult.Object)).Returns( _mockSearchResults.Object );