How to mock method with FilterExpression input parameter?

I’m trying to mock method using Moq that takes FilterExpression as an input parameter and I have no luck so far. When I run it is always returns null. If I pass NULL instead of FilterExpression it works.

Here is my code:

Repository

public interface ITestRepository
{
string Test(int id, FilterExpression fe);
}

public class TestRepository : ITestRepository
{
        public stringTest(int id, FilterExpression fe)
        {
            throw new NotImplementedException();
        }
}

Unit Test

var testMock = new Mock<ITestRepository>();
var fe = new FilterExpression();
var result = “Hello World”;
var  id = 1;

//DOESN’T WORK
testMock.Setup(r => r.Test(id, filterExpression)).Returns(result);

//WORKS
testMock.Setup(r => r.Test(id, null)).Returns(result);

//Test
[TestMethod]
public void test()
{
    var fe = new FilterExpression();
    var id = 1;        
    _testRepository.Test(id, fe);
    _testRepository.Test(id, null);       
}