From 14f7a469f19c5f17ba07b003c0e15e73c1a75ddc Mon Sep 17 00:00:00 2001
From: Dima Z <dmitry.zabello@gmail.com>
Date: Mon, 9 Feb 2026 18:44:34 -0800
Subject: [PATCH 1/2] Add JSONSerializer string check

---
 src/serialization.spec.ts | 6 ++++++
 src/serialization.ts      | 9 ++++++++-
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/src/serialization.spec.ts b/src/serialization.spec.ts
index a878604..fca5f58 100644
--- a/src/serialization.spec.ts
+++ b/src/serialization.spec.ts
@@ -53,6 +53,12 @@ describe('JSONSerializer', () => {
       ),
     );
   });
+
+  it('should throw an error when serialization result is undefined', () => {
+    expect(() => serializer.serialize(undefined)).toThrow(
+      new SerializerError('Not JSON serializable'),
+    );
+  });
 });
 
 describe('DefaultErrorSerializer', () => {
diff --git a/src/serialization.ts b/src/serialization.ts
index 1353fed..e5f0f4e 100644
--- a/src/serialization.ts
+++ b/src/serialization.ts
@@ -64,8 +64,15 @@ export class JSONSerializer<T> implements Serializer<T> {
   serialize(value: T): string {
     try {
       // Assumes the value is already in a JSON serializable format.
-      return JSON.stringify(value);
+      const result = JSON.stringify(value);
+      if (typeof result !== 'string') {
+        throw new SerializerError('Not JSON serializable');
+      }
+      return result;
     } catch (err) {
+      if (err instanceof SerializerError) {
+        throw err;
+      }
       throw new SerializerError('Not JSON serializable', err);
     }
   }

From d9290cc6e7d67d9bbee336aa239ce52425e29daf Mon Sep 17 00:00:00 2001
From: Dima Z <dmitry.zabello@gmail.com>
Date: Mon, 9 Feb 2026 18:49:09 -0800
Subject: [PATCH 2/2] Add SerializerError for nonstring

---
 src/executor.ts | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/executor.ts b/src/executor.ts
index 54d9386..7787c46 100644
--- a/src/executor.ts
+++ b/src/executor.ts
@@ -331,6 +331,10 @@ export class IdempotentExecutor {
           type: 'error',
           error: errorSerializer.serialize(value),
         });
+      } else if (value === undefined) {
+        await this.cache.set(cacheKey, {
+          type: 'value',
+        });
       } else {
         await this.cache.set(cacheKey, {
           type: 'value',