diff --git a/src/server/webStandardStreamableHttp.ts b/src/server/webStandardStreamableHttp.ts
index 1f528427c8..c37e0854fd 100644
--- a/src/server/webStandardStreamableHttp.ts
+++ b/src/server/webStandardStreamableHttp.ts
@@ -621,7 +621,13 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
             };
 
             let rawMessage;
-            if (options?.parsedBody !== undefined) {
+            // Treat both `undefined` and `null` as "no pre-parsed body". Some
+            // body parsers (notably the synthetic req/res objects produced by
+            // serverless-express on AWS Lambda Function URLs) set `req.body`
+            // to `null` after draining the request stream. Without this guard
+            // the `null` would be passed straight to JSONRPCMessageSchema.parse
+            // and fail with an opaque Zod error. See issue #1417.
+            if (options?.parsedBody !== undefined && options.parsedBody !== null) {
                 rawMessage = options.parsedBody;
             } else {
                 try {
diff --git a/test/server/streamableHttp.test.ts b/test/server/streamableHttp.test.ts
index 4a4f7d8248..3862fdf282 100644
--- a/test/server/streamableHttp.test.ts
+++ b/test/server/streamableHttp.test.ts
@@ -1292,6 +1292,46 @@ describe.each(zodTestMatrix)('$zodVersionLabel', (entry: ZodMatrixEntry) => {
         });
     });
 
+    // Regression test for #1417: serverless-express on AWS Lambda Function URLs
+    // drains the request stream and sets req.body to `null` rather than leaving
+    // it undefined. The transport should treat this the same as "no pre-parsed
+    // body" and fall through to reading the body itself, not pass `null` to the
+    // JSON-RPC parser.
+    describe('StreamableHTTPServerTransport with parsedBody = null (Lambda regression #1417)', () => {
+        let server: Server;
+        let transport: StreamableHTTPServerTransport;
+        let baseUrl: URL;
+
+        beforeEach(async () => {
+            const result = await createTestServer({
+                customRequestHandler: async (req, res) => {
+                    // Mimic serverless-express: pass `null` explicitly as parsedBody.
+                    await transport.handleRequest(req, res, null);
+                },
+                sessionIdGenerator: () => randomUUID()
+            });
+
+            server = result.server;
+            transport = result.transport;
+            baseUrl = result.baseUrl;
+        });
+
+        afterEach(async () => {
+            await stopTestServer({ server, transport });
+        });
+
+        it('parses the request body when parsedBody is null', async () => {
+            const response = await sendPostRequest(baseUrl, TEST_MESSAGES.initialize);
+
+            expect(response.status).toBe(200);
+            expect(response.headers.get('mcp-session-id')).toBeTruthy();
+
+            const text = await readSSEEvent(response);
+            expect(text).toContain('"id":"init-1"');
+            expect(text).toContain('"protocolVersion"');
+        });
+    });
+
     // Test resumability support
     describe('StreamableHTTPServerTransport with resumability', () => {
         let server: Server;