diff --git a/src/ApplicationCore/ApplicationCore.csproj b/src/ApplicationCore/ApplicationCore.csproj
index e0c49c1..f8d7d9a 100644
--- a/src/ApplicationCore/ApplicationCore.csproj
+++ b/src/ApplicationCore/ApplicationCore.csproj
@@ -5,6 +5,7 @@
   </PropertyGroup>
 
   <ItemGroup>
+    <PackageReference Include="Ardalis.GuardClauses" Version="1.1.1" />
     <PackageReference Include="System.Security.Claims" Version="4.3.0" />
   </ItemGroup>
 
diff --git a/src/ApplicationCore/Exceptions/BasketNotFoundException.cs b/src/ApplicationCore/Exceptions/BasketNotFoundException.cs
new file mode 100644
index 0000000..7a3f804
--- /dev/null
+++ b/src/ApplicationCore/Exceptions/BasketNotFoundException.cs
@@ -0,0 +1,23 @@
+using System;
+
+namespace ApplicationCore.Exceptions
+{
+    public class BasketNotFoundException : Exception
+    {
+        public BasketNotFoundException(int basketId) : base($"No basket found with id {basketId}")
+        {
+        }
+
+        protected BasketNotFoundException(System.Runtime.Serialization.SerializationInfo info, System.Runtime.Serialization.StreamingContext context) : base(info, context)
+        {
+        }
+
+        public BasketNotFoundException(string message) : base(message)
+        {
+        }
+
+        public BasketNotFoundException(string message, Exception innerException) : base(message, innerException)
+        {
+        }
+    }
+}
diff --git a/src/ApplicationCore/Exceptions/GuardExtensions.cs b/src/ApplicationCore/Exceptions/GuardExtensions.cs
new file mode 100644
index 0000000..0c840a0
--- /dev/null
+++ b/src/ApplicationCore/Exceptions/GuardExtensions.cs
@@ -0,0 +1,14 @@
+using ApplicationCore.Exceptions;
+using Microsoft.eShopWeb.ApplicationCore.Entities;
+
+namespace Ardalis.GuardClauses
+{
+    public static class FooGuard
+    {
+        public static void NullBasket(this IGuardClause guardClause, int basketId, Basket basket)
+        {
+            if (basket == null)
+                throw new BasketNotFoundException(basketId);
+        }
+    }
+}
\ No newline at end of file
diff --git a/src/ApplicationCore/Interfaces/IRepository.cs b/src/ApplicationCore/Interfaces/IRepository.cs
index a448152..9099188 100644
--- a/src/ApplicationCore/Interfaces/IRepository.cs
+++ b/src/ApplicationCore/Interfaces/IRepository.cs
@@ -1,12 +1,12 @@
 using Microsoft.eShopWeb.ApplicationCore.Entities;
 using System.Collections.Generic;
-using System.Threading.Tasks;
 
 namespace ApplicationCore.Interfaces
 {
     public interface IRepository<T> where T : BaseEntity
     {
         T GetById(int id);
+        T GetSingleBySpec(ISpecification<T> spec);
         IEnumerable<T> ListAll();
         IEnumerable<T> List(ISpecification<T> spec);
         T Add(T entity);
diff --git a/src/ApplicationCore/Services/BasketService.cs b/src/ApplicationCore/Services/BasketService.cs
index 209886d..f189812 100644
--- a/src/ApplicationCore/Services/BasketService.cs
+++ b/src/ApplicationCore/Services/BasketService.cs
@@ -4,6 +4,7 @@ using System.Collections.Generic;
 using ApplicationCore.Specifications;
 using Microsoft.eShopWeb.ApplicationCore.Entities;
 using System.Linq;
+using Ardalis.GuardClauses;
 
 namespace ApplicationCore.Services
 {
@@ -43,6 +44,7 @@ namespace ApplicationCore.Services
 
         public async Task<int> GetBasketItemCountAsync(string userName)
         {
+            Guard.Against.NullOrEmpty(userName, nameof(userName));
             var basketSpec = new BasketWithItemsSpecification(userName);
             var basket = (await _basketRepository.ListAsync(basketSpec)).FirstOrDefault();
             if (basket == null)
@@ -57,7 +59,9 @@ namespace ApplicationCore.Services
 
         public async Task SetQuantities(int basketId, Dictionary<string, int> quantities)
         {
+            Guard.Against.Null(quantities, nameof(quantities));
             var basket = await _basketRepository.GetByIdAsync(basketId);
+            Guard.Against.NullBasket(basketId, basket);
             foreach (var item in basket.Items)
             {
                 if (quantities.TryGetValue(item.Id.ToString(), out var quantity))
@@ -71,6 +75,8 @@ namespace ApplicationCore.Services
 
         public async Task TransferBasketAsync(string anonymousId, string userName)
         {
+            Guard.Against.NullOrEmpty(anonymousId, nameof(anonymousId));
+            Guard.Against.NullOrEmpty(userName, nameof(userName));
             var basketSpec = new BasketWithItemsSpecification(anonymousId);
             var basket = (await _basketRepository.ListAsync(basketSpec)).FirstOrDefault();
             if (basket == null) return;
diff --git a/src/ApplicationCore/Services/OrderService.cs b/src/ApplicationCore/Services/OrderService.cs
index e666182..162e289 100644
--- a/src/ApplicationCore/Services/OrderService.cs
+++ b/src/ApplicationCore/Services/OrderService.cs
@@ -3,6 +3,7 @@ using ApplicationCore.Entities.OrderAggregate;
 using System.Threading.Tasks;
 using Microsoft.eShopWeb.ApplicationCore.Entities;
 using System.Collections.Generic;
+using Ardalis.GuardClauses;
 
 namespace ApplicationCore.Services
 {
@@ -24,6 +25,7 @@ namespace ApplicationCore.Services
         public async Task CreateOrderAsync(int basketId, Address shippingAddress)
         {
             var basket = await _basketRepository.GetByIdAsync(basketId);
+            Guard.Against.NullBasket(basketId, basket);
             var items = new List<OrderItem>();
             foreach (var item in basket.Items)
             {
diff --git a/src/Infrastructure/Data/EfRepository.cs b/src/Infrastructure/Data/EfRepository.cs
index b0cecfa..47fe8a2 100644
--- a/src/Infrastructure/Data/EfRepository.cs
+++ b/src/Infrastructure/Data/EfRepository.cs
@@ -26,6 +26,12 @@ namespace Infrastructure.Data
             return _dbContext.Set<T>().Find(id);
         }
 
+        public T GetSingleBySpec(ISpecification<T> spec)
+        {
+            return List(spec).FirstOrDefault();
+        }
+
+
         public virtual async Task<T> GetByIdAsync(int id)
         {
             return await _dbContext.Set<T>().FindAsync(id);
diff --git a/src/Infrastructure/Logging/LoggerAdapter.cs b/src/Infrastructure/Logging/LoggerAdapter.cs
index 93b5447..240c5b1 100644
--- a/src/Infrastructure/Logging/LoggerAdapter.cs
+++ b/src/Infrastructure/Logging/LoggerAdapter.cs
@@ -15,6 +15,7 @@ namespace Infrastructure.Logging
         {
             _logger.LogWarning(message, args);
         }
+
         public void LogInformation(string message, params object[] args)
         {
             _logger.LogInformation(message, args);
diff --git a/tests/UnitTests/ApplicationCore/Services/BasketServiceTests/SetQuantities.cs b/tests/UnitTests/ApplicationCore/Services/BasketServiceTests/SetQuantities.cs
new file mode 100644
index 0000000..8e98009
--- /dev/null
+++ b/tests/UnitTests/ApplicationCore/Services/BasketServiceTests/SetQuantities.cs
@@ -0,0 +1,40 @@
+using ApplicationCore.Exceptions;
+using ApplicationCore.Interfaces;
+using ApplicationCore.Services;
+using Microsoft.eShopWeb.ApplicationCore.Entities;
+using Moq;
+using System;
+using Xunit;
+
+namespace UnitTests.ApplicationCore.Services.BasketServiceTests
+{
+    public class SetQuantities
+    {
+        private int _invalidId = -1;
+        private Mock<IAsyncRepository<Basket>> _mockBasketRepo;
+
+        public SetQuantities()
+        {
+            _mockBasketRepo = new Mock<IAsyncRepository<Basket>>();
+        }
+
+        [Fact]
+        public async void ThrowsGivenInvalidBasketId()
+        {
+            var basketService = new BasketService(_mockBasketRepo.Object, null, null, null);
+
+            await Assert.ThrowsAsync<BasketNotFoundException>(async () =>
+                await basketService.SetQuantities(_invalidId, new System.Collections.Generic.Dictionary<string, int>()));
+        }
+
+        [Fact]
+        public async void ThrowsGivenNullQuantities()
+        {
+            var basketService = new BasketService(null, null, null, null);
+
+            await Assert.ThrowsAsync<ArgumentNullException>(async () =>
+                await basketService.SetQuantities(123, null));
+        }
+
+    }
+}
diff --git a/tests/UnitTests/ApplicationCore/Services/BasketServiceTests/TransferBasket.cs b/tests/UnitTests/ApplicationCore/Services/BasketServiceTests/TransferBasket.cs
new file mode 100644
index 0000000..f284f56
--- /dev/null
+++ b/tests/UnitTests/ApplicationCore/Services/BasketServiceTests/TransferBasket.cs
@@ -0,0 +1,25 @@
+using ApplicationCore.Services;
+using System;
+using Xunit;
+
+namespace UnitTests.ApplicationCore.Services.BasketServiceTests
+{
+    public class TransferBasket
+    {
+        [Fact]
+        public async void ThrowsGivenNullAnonymousId()
+        {
+            var basketService = new BasketService(null, null, null, null);
+
+            await Assert.ThrowsAsync<ArgumentNullException>(async () => await basketService.TransferBasketAsync(null, "steve"));
+        }
+
+        [Fact]
+        public async void ThrowsGivenNullUserId()
+        {
+            var basketService = new BasketService(null, null, null, null);
+
+            await Assert.ThrowsAsync<ArgumentNullException>(async () => await basketService.TransferBasketAsync("abcdefg", null));
+        }
+    }
+}