How to Unit Test a Method that Use Mapper in .Net Core Web API Project

I am working on a .net core web api project and writing some unit tests for my methods. One of my test cases is unable to calculate expected output.

In test project, I have the following code:

_mockBaseDbContext.Setup(c => c.Transactions).Returns(mockTransactions.Object);
var actualResult = await _service.Get(transactionId);

In the web api, I have the corresponding method as follows:

public async Task<TransactionViewModel> Get(Guid id)
{
     var transaction = await GetById(id);
     var result = _mapper.Map<TransactionViewModel>(transaction);
     return result;
}

So, when i run the test case, it finds the transaction var transaction = await GetById(id); //works fine but it just can not map the Transaction to TransactionViewModel

_mapper.Map<TransactionViewModel>(transaction); //returns null

I have the mapping profile in startup.cs and it works when I run the web api, I mean I have written an endpoint that takes guid as a parameter and call my meyhod, then it returns the transaction view model without any trouble.

CreateMap<Transaction, TransactionViewModel>()
                .ForMember(dest => dest.Client, opt => opt.MapFrom(src => src.Client))
                .ForMember(dest => dest.ShopId, opt => opt.MapFrom(src => src.ShopId));

So my question is that is there a way to get the view model that is returned from my Get method? Thanks in advance.