diff --git a/OptimizelySDK.Tests/OptimizelyUserContextTest.cs b/OptimizelySDK.Tests/OptimizelyUserContextTest.cs
index 21ae10db..49b3a903 100644
--- a/OptimizelySDK.Tests/OptimizelyUserContextTest.cs
+++ b/OptimizelySDK.Tests/OptimizelyUserContextTest.cs
@@ -405,6 +405,23 @@ public void DecideInvalidFlagKey()
             Assert.IsTrue(TestData.CompareObjects(decision, decisionExpected));
         }
 
+        [Test]
+        public void DecideNullFlagKey()
+        {
+            var user = Optimizely.CreateUserContext(UserID);
+            user.SetAttribute("browser_type", "chrome");
+
+            var decisionExpected = OptimizelyDecision.NewErrorDecision(
+                null,
+                user,
+                DecisionMessage.Reason(DecisionMessage.FLAG_KEY_INVALID, "null"),
+                ErrorHandlerMock.Object,
+                LoggerMock.Object);
+            var decision = user.Decide(null);
+
+            Assert.IsTrue(TestData.CompareObjects(decision, decisionExpected));
+        }
+
         [Test]
         public void DecideWhenConfigIsNull()
         {
diff --git a/OptimizelySDK/Optimizely.cs b/OptimizelySDK/Optimizely.cs
index 9040ea17..4e0a0bce 100644
--- a/OptimizelySDK/Optimizely.cs
+++ b/OptimizelySDK/Optimizely.cs
@@ -871,6 +871,13 @@ OptimizelyDecideOption[] options
                     ErrorHandler, Logger);
             }
 
+            if (key == null)
+            {
+                return OptimizelyDecision.NewErrorDecision(key, user,
+                    DecisionMessage.Reason(DecisionMessage.FLAG_KEY_INVALID, "null"),
+                    ErrorHandler, Logger);
+            }
+
             var allOptions = GetAllOptions(options).
                 Where(opt => opt != OptimizelyDecideOption.ENABLED_FLAGS_ONLY).
                 ToArray();