diff --git a/DripDotNet/Resources/DripSubscriber.cs b/DripDotNet/Resources/DripSubscriber.cs
index 9085c54..53fca7a 100644
--- a/DripDotNet/Resources/DripSubscriber.cs
+++ b/DripDotNet/Resources/DripSubscriber.cs
@@ -39,6 +39,11 @@ public class DripSubscriber
         /// </summary>
         public string Email { get; set; }
 
+        /// <summary>
+        /// The subscriber's user id (a unique identifier for the user, such as a primary key).
+        /// </summary>
+        public string UserId { get; set; }
+
         /// <summary>
         /// The subscriber's time zone (in Olsen format). Defaults to Etc/UTC
         /// </summary>
@@ -108,6 +113,11 @@ public class ModifyDripSubscriberRequest
         /// </summary>
         public string Email { get; set; }
 
+        /// <summary>
+        /// The subscriber's user id (a unique identifier for the user, such as a primary key).
+        /// </summary>
+        public string UserId { get; set; }
+
         /// <summary>
         /// The subscriber's time zone (in Olsen format). Defaults to Etc/UTC
         /// </summary>
diff --git a/DripDotNetTests/CampaignTests.cs b/DripDotNetTests/CampaignTests.cs
index fa432db..b721eaa 100644
--- a/DripDotNetTests/CampaignTests.cs
+++ b/DripDotNetTests/CampaignTests.cs
@@ -35,7 +35,7 @@ namespace DripDotNetTests
     public class CampaignTests: IClassFixture<DripClientFixture>, IClassFixture<SubscriberFactoryFixture>
     {
         //TODO: put this in a configuration file
-        const string TestCampaignId = "3057996"; //This is a test campaign in my test account
+        const string TestCampaignId = "3057996"; //This is a test campaign in my test account. Replace this with a Campaign Id from your account before running tests.
 
         DripClientFixture dripClientFixture;
         SubscriberFactoryFixture subscriberFactoryFixture;
diff --git a/DripDotNetTests/SubscriberFactoryFixture.cs b/DripDotNetTests/SubscriberFactoryFixture.cs
index fc9b7e9..db91dce 100644
--- a/DripDotNetTests/SubscriberFactoryFixture.cs
+++ b/DripDotNetTests/SubscriberFactoryFixture.cs
@@ -44,6 +44,7 @@ public ModifyDripSubscriberRequest CreateComplexUniqueModifyDripSubscriber()
             var result = new ModifyDripSubscriberRequest
             {
                 Email = GetRandomEmailAddress(),
+                UserId = IntUtil.Random(1001,9999).ToString(),
                 CustomFields = new Dictionary<string, string>
                 { 
                     {"id", Guid.NewGuid().ToString("n")},
@@ -70,4 +71,23 @@ public void Dispose()
         {
         }
     }
+
+    /*
+    This support class is used to generate a random integer for the UserId field.
+    */
+    public static class IntUtil
+    {
+        private static Random random;
+
+        private static void Init()
+        {
+            if (random == null) random = new Random();
+        }
+
+        public static int Random(int min, int max)
+        {
+            Init();
+            return random.Next(min, max);
+        }
+    }
 }
diff --git a/DripDotNetTests/SubscriberTests.cs b/DripDotNetTests/SubscriberTests.cs
index 48ad3d1..968cd80 100644
--- a/DripDotNetTests/SubscriberTests.cs
+++ b/DripDotNetTests/SubscriberTests.cs
@@ -56,6 +56,7 @@ public void CanCreateUpdateAndRetrieveSubscriber()
             Assert.Equal(1, result.Subscribers.Count);
 
             var actual = result.Subscribers.First();
+            Assert.Equal(expected.UserId, actual.UserId);
             DripAssert.Equal(expected.CustomFields, actual.CustomFields);
             DripAssert.ContainsSameItems(expected.Tags, actual.Tags);
 
@@ -90,6 +91,7 @@ public async Task CanCreateUpdateAndRetrieveSubscriberAsync()
 
             var actual = result.Subscribers.First();
             Assert.Equal(expected.Email, actual.Email);
+            Assert.Equal(expected.UserId, actual.UserId);
             DripAssert.Equal(expected.CustomFields, actual.CustomFields);
             DripAssert.ContainsSameItems(expected.Tags, actual.Tags);