Unit Test Using Moq

I am unit-testing an async method that returns a List<T>. This method has a dependency on a mapping class/interface. In my unit-test, I am mocking the mapping class using moq. The test runs okay, and the returned list has items, but the values of the items is null. I think the problem is because I haven't stubbed-out the mapping classes methods properly. I don't have a lot of experience with testing, so any guidance is appreciated.

Test Method:

[TestMethod]
[TestCategory("CSR.Data.Tests.Services.ServiceSearchTest")]
public void SearchAccount()
    {
        // Arrange                 
        var mapper = new Mock<CSR.Data.Mapping.Interfaces.IMapper<Account, AccountDTO>>();

        mapper.Setup(i => i.Initialize());
        mapper.Setup(i => i.ToDomain(It.IsAny<AccountSearchResult>())).Returns(It.IsAny<Account>);
        mapper.Setup(i => i.DomainToDto(It.IsAny<Account>())).Returns(It.IsAny<AccountDTO>);

        var service = new ServiceSearch(null,mapper.Object);         
        string accountNumber = "123";
        string accountName = "";

        // Act 
        var results = service.SearchAccount(accountNumber, accountName);            

       // Assert
        Assert.IsTrue(results.Result.Count >= 1);
        }

Method/Class That I'm Testing:

    public class ServiceSearch : IServiceSearch
    {
    public ServiceSearch(IMapper<Claim, ClaimDTO> claimMapper, IMapper<Account, AccountDTO> accountMapper)
            {
                _claimMapper = claimMapper;
                _accountMapper = accountMapper;
            }

    public async Task<List<AccountDTO>> SearchAccount(string accountNumber, string accountName)
            {
                var accounts = new List<Account>();
                var accountDTOs = new List<AccountDTO>();
                var results = await Task.Run(() => base.AccountSearch(accountNumber, accountName).Result);

                if (results != null && results.Count > 0)
                {
                    //Map DH to Domain
                    _accountMapper.Initialize();

                    foreach (AccountSearchResult result in results)
                    {
                        accounts.Add(_accountMapper.ToDomain(result));
                    }

                    //Map Domain to DTO
                    foreach (Account account in accounts)
                    {
                        accountDTOs.Add(_accountMapper.DomainToDto(account));
                    }
                }
                return accountDTOs;
            }
}