diff --git a/Sift/Core/Client.cs b/Sift/Core/Client.cs
index a7d8da5c..be45012c 100644
--- a/Sift/Core/Client.cs
+++ b/Sift/Core/Client.cs
@@ -15,11 +15,22 @@ public class Client : IDisposable
 
         public Client(String apiKey)
         {
+            if (string.IsNullOrWhiteSpace(apiKey))
+            {
+                throw new ArgumentNullException(nameof(apiKey));
+            }
+
             this.apiKey = apiKey;
             this.http = new HttpClient();
         }
 
-        public Client(String apiKey, HttpClient http) {
+        public Client(String apiKey, HttpClient http)
+        {
+            if (string.IsNullOrWhiteSpace(apiKey))
+            {
+                throw new ArgumentNullException(nameof(apiKey));
+            }
+
             this.apiKey = apiKey;
             this.http = http;
         }
diff --git a/Test/ClientTest.cs b/Test/ClientTest.cs
new file mode 100644
index 00000000..fa764b3c
--- /dev/null
+++ b/Test/ClientTest.cs
@@ -0,0 +1,32 @@
+using Sift;
+using Xunit;
+using System.Net.Http;
+
+namespace Test
+{
+    public class ClientTest
+    {
+        [Fact]
+        public void TestFailsToCreateClientWithAPIKeyNull()
+        {
+            Assert.Throws<ArgumentNullException>(
+                () => new Client(null)
+            );
+            Assert.Throws<ArgumentNullException>(
+                () => new Client(null, new HttpClient())
+            );
+        }
+
+        [Fact]
+        public void TestFailsToCreateClientWithEmptyAPIKey()
+        {
+            Assert.Throws<ArgumentNullException>(
+                () => new Client("")
+            );
+            Assert.Throws<ArgumentNullException>(
+                () => new Client("", new HttpClient())
+            );
+        }
+    }
+
+}