diff --git a/packages/durabletask-js/src/testing/test-client.ts b/packages/durabletask-js/src/testing/test-client.ts
index f7af0a9..5b8f448 100644
--- a/packages/durabletask-js/src/testing/test-client.ts
+++ b/packages/durabletask-js/src/testing/test-client.ts
@@ -92,7 +92,9 @@ export class TestOrchestrationClient {
    * Raises an event to an orchestration.
    */
   async raiseOrchestrationEvent(instanceId: string, eventName: string, data: any = null): Promise<void> {
-    const encodedData = data !== null ? JSON.stringify(data) : undefined;
+    // Always serialize data — including null — to match TaskHubGrpcClient behavior.
+    // The real client unconditionally calls JSON.stringify(data), which turns null into "null".
+    const encodedData = JSON.stringify(data);
     this.backend.raiseEvent(instanceId, eventName, encodedData);
   }
 
@@ -100,7 +102,9 @@ export class TestOrchestrationClient {
    * Terminates an orchestration.
    */
   async terminateOrchestration(instanceId: string, output: any = null): Promise<void> {
-    const encodedOutput = output !== null ? JSON.stringify(output) : undefined;
+    // Always serialize output — including null — to match TaskHubGrpcClient behavior.
+    // The real client unconditionally calls JSON.stringify(output), which turns null into "null".
+    const encodedOutput = JSON.stringify(output);
     this.backend.terminate(instanceId, encodedOutput);
   }
 
diff --git a/packages/durabletask-js/test/test-client-serialization.spec.ts b/packages/durabletask-js/test/test-client-serialization.spec.ts
new file mode 100644
index 0000000..32f65c1
--- /dev/null
+++ b/packages/durabletask-js/test/test-client-serialization.spec.ts
@@ -0,0 +1,191 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+import {
+  InMemoryOrchestrationBackend,
+  TestOrchestrationClient,
+  TestOrchestrationWorker,
+  OrchestrationStatus,
+  OrchestrationContext,
+  TOrchestrator,
+} from "../src";
+
+/**
+ * Tests that TestOrchestrationClient serializes null values the same way as the
+ * real TaskHubGrpcClient.
+ *
+ * The real client unconditionally calls JSON.stringify(data) even when the value
+ * is null, which produces the string "null". The test client must match this
+ * behavior so that orchestrations tested with the in-memory backend receive the
+ * same values they would in production.
+ */
+describe("TestOrchestrationClient null serialization", () => {
+  let backend: InMemoryOrchestrationBackend;
+  let client: TestOrchestrationClient;
+  let worker: TestOrchestrationWorker;
+
+  beforeEach(async () => {
+    backend = new InMemoryOrchestrationBackend();
+    client = new TestOrchestrationClient(backend);
+    worker = new TestOrchestrationWorker(backend);
+  });
+
+  afterEach(async () => {
+    if (worker) {
+      try {
+        await worker.stop();
+      } catch {
+        // Ignore if not running
+      }
+    }
+    backend.reset();
+  });
+
+  it("raiseOrchestrationEvent with null data should deliver null (not undefined)", async () => {
+    let receivedValue: any = "sentinel";
+
+    const orchestrator: TOrchestrator = async function* (ctx: OrchestrationContext): any {
+      receivedValue = yield ctx.waitForExternalEvent("my_event");
+      return receivedValue;
+    };
+
+    worker.addOrchestrator(orchestrator);
+    await worker.start();
+
+    const id = await client.scheduleNewOrchestration(orchestrator);
+    await client.waitForOrchestrationStart(id, false, 5);
+
+    // Raise event with no data (defaults to null)
+    await client.raiseOrchestrationEvent(id, "my_event");
+
+    const state = await client.waitForOrchestrationCompletion(id, true, 10);
+
+    expect(state).toBeDefined();
+    expect(state?.runtimeStatus).toEqual(OrchestrationStatus.COMPLETED);
+    // The orchestrator should receive null — the same value the real client delivers.
+    // Before this fix, the test client would deliver undefined instead.
+    expect(receivedValue).toBeNull();
+    expect(state?.serializedOutput).toEqual("null");
+  });
+
+  it("raiseOrchestrationEvent with explicit null should deliver null", async () => {
+    let receivedValue: any = "sentinel";
+
+    const orchestrator: TOrchestrator = async function* (ctx: OrchestrationContext): any {
+      receivedValue = yield ctx.waitForExternalEvent("my_event");
+      return receivedValue;
+    };
+
+    worker.addOrchestrator(orchestrator);
+    await worker.start();
+
+    const id = await client.scheduleNewOrchestration(orchestrator);
+    await client.waitForOrchestrationStart(id, false, 5);
+
+    // Raise event with explicit null
+    await client.raiseOrchestrationEvent(id, "my_event", null);
+
+    const state = await client.waitForOrchestrationCompletion(id, true, 10);
+
+    expect(state).toBeDefined();
+    expect(state?.runtimeStatus).toEqual(OrchestrationStatus.COMPLETED);
+    expect(receivedValue).toBeNull();
+  });
+
+  it("raiseOrchestrationEvent with non-null data should work normally", async () => {
+    let receivedValue: any = "sentinel";
+
+    const orchestrator: TOrchestrator = async function* (ctx: OrchestrationContext): any {
+      receivedValue = yield ctx.waitForExternalEvent("my_event");
+      return receivedValue;
+    };
+
+    worker.addOrchestrator(orchestrator);
+    await worker.start();
+
+    const id = await client.scheduleNewOrchestration(orchestrator);
+    await client.waitForOrchestrationStart(id, false, 5);
+
+    await client.raiseOrchestrationEvent(id, "my_event", { key: "value" });
+
+    const state = await client.waitForOrchestrationCompletion(id, true, 10);
+
+    expect(state).toBeDefined();
+    expect(state?.runtimeStatus).toEqual(OrchestrationStatus.COMPLETED);
+    expect(receivedValue).toEqual({ key: "value" });
+  });
+
+  it("raiseOrchestrationEvent with falsy values should serialize them", async () => {
+    const receivedValues: any[] = [];
+
+    const orchestrator: TOrchestrator = async function* (ctx: OrchestrationContext): any {
+      receivedValues.push(yield ctx.waitForExternalEvent("e1"));
+      receivedValues.push(yield ctx.waitForExternalEvent("e2"));
+      receivedValues.push(yield ctx.waitForExternalEvent("e3"));
+      return receivedValues;
+    };
+
+    worker.addOrchestrator(orchestrator);
+    await worker.start();
+
+    const id = await client.scheduleNewOrchestration(orchestrator);
+    await client.waitForOrchestrationStart(id, false, 5);
+
+    // 0, false, and "" are all falsy but valid JSON values
+    await client.raiseOrchestrationEvent(id, "e1", 0);
+    await client.raiseOrchestrationEvent(id, "e2", false);
+    await client.raiseOrchestrationEvent(id, "e3", "");
+
+    const state = await client.waitForOrchestrationCompletion(id, true, 10);
+
+    expect(state).toBeDefined();
+    expect(state?.runtimeStatus).toEqual(OrchestrationStatus.COMPLETED);
+    expect(receivedValues[0]).toBe(0);
+    expect(receivedValues[1]).toBe(false);
+    expect(receivedValues[2]).toBe("");
+  });
+
+  it("terminateOrchestration with null output should store null (not undefined)", async () => {
+    const orchestrator: TOrchestrator = async function* (ctx: OrchestrationContext): any {
+      yield ctx.waitForExternalEvent("never");
+      return "never reached";
+    };
+
+    worker.addOrchestrator(orchestrator);
+    await worker.start();
+
+    const id = await client.scheduleNewOrchestration(orchestrator);
+    await client.waitForOrchestrationStart(id, false, 5);
+
+    // Terminate with no output (defaults to null)
+    await client.terminateOrchestration(id);
+
+    const state = await client.waitForOrchestrationCompletion(id, true, 10);
+
+    expect(state).toBeDefined();
+    expect(state?.runtimeStatus).toEqual(OrchestrationStatus.TERMINATED);
+    // The real client stores "null" as the serialized output, not undefined
+    expect(state?.serializedOutput).toEqual("null");
+  });
+
+  it("terminateOrchestration with explicit output should serialize it", async () => {
+    const orchestrator: TOrchestrator = async function* (ctx: OrchestrationContext): any {
+      yield ctx.waitForExternalEvent("never");
+      return "never reached";
+    };
+
+    worker.addOrchestrator(orchestrator);
+    await worker.start();
+
+    const id = await client.scheduleNewOrchestration(orchestrator);
+    await client.waitForOrchestrationStart(id, false, 5);
+
+    await client.terminateOrchestration(id, "stopped");
+
+    const state = await client.waitForOrchestrationCompletion(id, true, 10);
+
+    expect(state).toBeDefined();
+    expect(state?.runtimeStatus).toEqual(OrchestrationStatus.TERMINATED);
+    expect(state?.serializedOutput).toEqual(JSON.stringify("stopped"));
+  });
+});