Identity Error on UserName in Moq - asp.net

Question

I try to set an user object property in my unit test. This property is correctly passed to a custom user validator and was verified to be set during debugging. However, the IdentityResult still shows a blank property error.
I have this custom user validator:
public class CustomUserValidator : UserValidator<AppUser>
{
public override async Task<IdentityResult> ValidateAsync(UserManager<AppUser> userManager, AppUser user)
{
//Here, the user.UserName property is set from the unit test, but the result still has an error for a blank UserName property
IdentityResult result = await base.ValidateAsync(userManager, user);
List<IdentityError> errors = result.Succeeded ? new List<IdentityError>() : result.Errors.ToList();
if (user.Email.ToLower().EndsWith("#myDomain.com"))
{
return await Task.FromResult(IdentityResult.Success);
}
else
{
errors.Add(new IdentityError
{
Code = "EmailDomainError",
Description = "Only myDomain.com email addresses are allowed"
});
}
return errors.Count == 0 ? IdentityResult.Success : IdentityResult.Failed(errors.ToArray());
}
}
And I'm trying to use Moq and xUnit to create a test for the domain check on the email:
[Fact]
public async void Validate_User_Email()
{
//Arrange
var userManager = GetMockUserManager();
var customVal = new CustomUserValidator();
var user = Mock.Of<AppUser>(m => m.UserName == "joe" && m.Email == "joe#example.com");
//Act
//try to validate the user
IdentityResult result = await customVal.ValidateAsync(userManager.Object, user);
//Assert
//demonstrate that there are errors present
List<IdentityError> errors = result.Succeeded ? new List<IdentityError>() : result.Errors.ToList();
//Here there are 2 errors, one for a blank UserName and one (expected) error for the email address
Assert.Equal(1, errors.Count);
}