diff --git a/DevCycle.SDK.Server.Local.MSTests/DevCycleTest.cs b/DevCycle.SDK.Server.Local.MSTests/DevCycleTest.cs
index 7d4ba83..0ee39f0 100644
--- a/DevCycle.SDK.Server.Local.MSTests/DevCycleTest.cs
+++ b/DevCycle.SDK.Server.Local.MSTests/DevCycleTest.cs
@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Threading.Tasks;
 using DevCycle.SDK.Server.Local.Api;
 using DevCycle.SDK.Server.Common.Model;
@@ -233,6 +233,67 @@ public void Variable_NullUser_ThrowsException()
             });
         }
 
+        [TestMethod]
+        public void Variable_NullKey_ThrowsArgumentException()
+        {
+            // Reaching the WASM bucketing engine with a null/empty flag key
+            // triggers an internal abort() and corrupts the WASM heap. Match
+            // the Java/Python SDKs (and Cloud client) by failing fast with a
+            // clear ArgumentException before we ever enter WASM.
+            using DevCycleLocalClient api = DevCycleTestClient.getTestClient();
+            var user = new DevCycleUser("test_user");
+
+            Assert.Throws<ArgumentException>(() => api.Variable(user, null, true).Result);
+        }
+
+        [TestMethod]
+        public void Variable_EmptyKey_ThrowsArgumentException()
+        {
+            using DevCycleLocalClient api = DevCycleTestClient.getTestClient();
+            var user = new DevCycleUser("test_user");
+
+            Assert.Throws<ArgumentException>(() => api.Variable(user, "", true).Result);
+        }
+
+        [TestMethod]
+        public async Task VariableAsync_NullKey_ThrowsArgumentException()
+        {
+            using DevCycleLocalClient api = DevCycleTestClient.getTestClient();
+            var user = new DevCycleUser("test_user");
+
+            await Assert.ThrowsExactlyAsync<ArgumentException>(async () =>
+                await api.VariableAsync(user, null, true));
+        }
+
+        [TestMethod]
+        public async Task VariableAsync_EmptyKey_ThrowsArgumentException()
+        {
+            using DevCycleLocalClient api = DevCycleTestClient.getTestClient();
+            var user = new DevCycleUser("test_user");
+
+            await Assert.ThrowsExactlyAsync<ArgumentException>(async () =>
+                await api.VariableAsync(user, "", true));
+        }
+
+        [TestMethod]
+        public void Variable_NullDefaultValue_ThrowsArgumentNullException()
+        {
+            using DevCycleLocalClient api = DevCycleTestClient.getTestClient();
+            var user = new DevCycleUser("test_user");
+
+            Assert.Throws<ArgumentNullException>(() => api.Variable<string>(user, "some_key", null).Result);
+        }
+
+        [TestMethod]
+        public async Task VariableAsync_NullDefaultValue_ThrowsArgumentNullException()
+        {
+            using DevCycleLocalClient api = DevCycleTestClient.getTestClient();
+            var user = new DevCycleUser("test_user");
+
+            await Assert.ThrowsExactlyAsync<ArgumentNullException>(async () =>
+                await api.VariableAsync<string>(user, "some_key", null));
+        }
+
         [TestMethod]
         public void User_NullUserId_ThrowsException()
         {
diff --git a/DevCycle.SDK.Server.Local/Api/DevCycleLocalClient.cs b/DevCycle.SDK.Server.Local/Api/DevCycleLocalClient.cs
index 01cf24c..8d608c1 100644
--- a/DevCycle.SDK.Server.Local/Api/DevCycleLocalClient.cs
+++ b/DevCycle.SDK.Server.Local/Api/DevCycleLocalClient.cs
@@ -307,6 +307,16 @@ public override Task<Variable<T>> Variable<T>(DevCycleUser user, string key, T d
         {
             var requestUser = new DevCyclePopulatedUser(user);
 
+            if (string.IsNullOrEmpty(key))
+            {
+                throw new ArgumentException("key cannot be null or empty");
+            }
+
+            if (defaultValue == null)
+            {
+                throw new ArgumentNullException(nameof(defaultValue));
+            }
+
             if (!configManager.Initialized)
             {
                 logger.LogWarning("Variable called before DevCycleClient has initialized, returning default value");
@@ -366,6 +376,16 @@ public async Task<Variable<T>> VariableAsync<T>(DevCycleUser user, string key, T
         {
             var requestUser = new DevCyclePopulatedUser(user);
 
+            if (string.IsNullOrEmpty(key))
+            {
+                throw new ArgumentException("key cannot be null or empty");
+            }
+
+            if (defaultValue == null)
+            {
+                throw new ArgumentNullException(nameof(defaultValue));
+            }
+
             if (!configManager.Initialized)
             {
                 logger.LogWarning("Variable called before DevCycleClient has initialized, returning default value");