ASP.Net Web API Controller Unit Test POST

I am new to Unit Testing and am trying to create some Xunit tests for my Web API Controller's POST method.

Here is my Controller's POST method:

[HttpPost("")]
public async Task<IActionResult> CreateArea([FromBody] AreaForCreationDto area)
{
    // Check that the 'area' object parameter can be de-serialised to a AreaForCreationDto.
    if (area == null)
    {
        var message = "Could not de-serialise the request body to an AreaForCreationDto object";
        _logger.LogError(message);

        // Return an error 400.
        return BadRequest(message);
    }

    /*
     * ModelState.IsValid is determined by the attributes associated with the 
     * Data Annotations on the properties of the ViewModel.
     */
    if (!ModelState.IsValid)
    {
        // Return a response with a Status Code 422.
        return new UnprocessableEntityObjectResult(ModelState);
    };

    // Map a AreaForCreationDto object to a Area entity.
    var areaEntityToAdd = _mapper.Map<Area>(area);

    // Call the repository to add the new Area entity to the DbContext.
    _areaRepository.AddArea(areaEntityToAdd);

    // Save the new Area entity, added to the DbContext, to the SQL database.
    if (await _areaRepository.SaveChangesAsync())
    {
        // Note: AutoMapper maps the values of the properties from the areaEntityToAdd
        // to a new areaToReturn object.
        // This ensures that we don't expose our Area entity to the web browser.
        var areaToReturn = _mapper.Map<AreaDto>(areaEntityToAdd);

        // Return a 201 'created' response along with the location URL in the
        // response Header.
        return CreatedAtRoute("GetArea",
            new { id = areaToReturn.Id },
            areaToReturn);
    }
    else {
        // The save failed.
        var message = $"Could not save new Area {areaEntityToAdd.Id} to the database.";
        _logger.LogWarning(message);
        throw new Exception(message);
    };
}

The first Unit Test I have written is intended to ensure that on sending a POST request, with an object which can be de-serialised into an AreaForCreation object, the function returns a 201 CreatedAtRouteResult along with the new Area which has been created.

Here is the Xunit test:

[Fact]
public void ReturnAreaForCreateArea()
{
    //Arrange

    var _mockAreaRepository = new Mock<IAreaRepository>();
    _mockAreaRepository
        .Setup(x => x.AddArea(testArea));

    var _mockMapper = new Mock<IMapper>();
    _mockMapper
        .Setup(_ => _.Map<Area>(It.IsAny<AreaForCreationDto>()))
        .Returns(testArea);

    var _mockLogger = new Mock<ILogger<AreasController>>();
    var _sut = new AreasController(_mockAreaRepository.Object, _mockLogger.Object, _mockMapper.Object);

    // Act
    var result = _sut.CreateArea(testAreaForCreationDto);

    // Assert
    Assert.NotNull(result);
    var objectResult = Assert.IsType<CreatedAtRouteResult>(result);
    var model = Assert.IsAssignableFrom<AreaDto>(objectResult.Value);
    var areaDescription = model.Description;
    Assert.Equal("Test Area For Creation", areaDescription);
}

I am getting an exception thrown when the unit test tries to Assert.IsType<CreatedAtRouteResult>(result). Debugging revealed that the Controller was failing to save to the repository. My AreaRepository has the following AddArea function which does not return a value so I assume that my _mockAreaRepository does not require a Return condition set (could be wrong here).

Do I need to configure my mockAreasRepository for the outcome of calling SaveChangesAsync()?