diff --git a/src/workerd/api/events.c++ b/src/workerd/api/events.c++
index 744af834f33..34e18b57beb 100644
--- a/src/workerd/api/events.c++
+++ b/src/workerd/api/events.c++
@@ -72,7 +72,7 @@ MessageEvent::MessageEvent(jsg::Lock& js, kj::String type, Initializer initializ
           }),
       data(kj::mv(initializer.data).orDefault([&] { return jsg::JsRef(js, js.null()); })),
       lastEventId(kj::mv(initializer.lastEventId).orDefault(kj::String())),
-      maybeSource(kj::mv(initializer.source)),
+      maybeSource(kj::mv(initializer.source).orDefault(kj::none)),
       // Per the spec, origin defaults to the empty string for user-constructed events.
       maybeOrigin(kj::mv(initializer.origin)
                       .map([](jsg::USVString&& origin) -> kj::String { return kj::mv(origin); })
diff --git a/src/workerd/api/events.h b/src/workerd/api/events.h
index 07f7956a561..7eea774a62f 100644
--- a/src/workerd/api/events.h
+++ b/src/workerd/api/events.h
@@ -52,7 +52,7 @@ class MessageEvent final: public Event {
     jsg::Optional<jsg::JsRef<jsg::JsValue>> data;
     jsg::Optional<jsg::USVString> origin;
     jsg::Optional<kj::String> lastEventId;
-    jsg::Optional<jsg::Ref<MessagePort>> source;
+    jsg::Optional<kj::Maybe<jsg::Ref<MessagePort>>> source;
     jsg::Optional<kj::Array<jsg::Ref<MessagePort>>> ports;
 
     JSG_STRUCT(bubbles, cancelable, composed, data, origin, lastEventId, source, ports);
diff --git a/src/workerd/api/tests/events-test.js b/src/workerd/api/tests/events-test.js
index c2d53a1bb96..d83eb006ddd 100644
--- a/src/workerd/api/tests/events-test.js
+++ b/src/workerd/api/tests/events-test.js
@@ -782,6 +782,43 @@ export const messageEventSpecInit = {
   },
 };
 
+// MessageEventInit's source is nullable, so an event whose source reads as null can
+// serve as the init for a new event.
+export const messageEventNullSource = {
+  async test() {
+    strictEqual(new MessageEvent('message', { source: null }).source, null);
+
+    const original = new MessageEvent('message', {
+      data: 'x',
+      lastEventId: '7',
+    });
+    const copy = new MessageEvent(original.type, original);
+    strictEqual(copy.data, 'x');
+    strictEqual(copy.lastEventId, '7');
+    strictEqual(copy.source, null);
+
+    // Message events fired by a WebSocket have a null source as well.
+    const { 0: client, 1: server } = new WebSocketPair();
+    client.accept();
+    server.accept();
+    const { promise, resolve, reject } = Promise.withResolvers();
+    client.addEventListener('message', (event) => {
+      try {
+        resolve(new MessageEvent(event.type, event));
+      } catch (err) {
+        reject(err);
+      }
+    });
+    server.send('hello');
+    const cloned = await promise;
+    strictEqual(cloned.data, 'hello');
+    strictEqual(cloned.source, null);
+
+    client.close();
+    server.close();
+  },
+};
+
 // CloseEventInit supports the common EventInit members.
 export const closeEventSpecInit = {
   test() {
diff --git a/types/generated-snapshot/experimental/index.d.ts b/types/generated-snapshot/experimental/index.d.ts
index 867c301208b..3bd84f5ac64 100755
--- a/types/generated-snapshot/experimental/index.d.ts
+++ b/types/generated-snapshot/experimental/index.d.ts
@@ -1752,7 +1752,7 @@ interface MessageEventInit {
   data?: any;
   origin?: string;
   lastEventId?: string;
-  source?: MessagePort;
+  source?: MessagePort | null;
   ports?: MessagePort[];
 }
 /**
diff --git a/types/generated-snapshot/experimental/index.ts b/types/generated-snapshot/experimental/index.ts
index b4b45c6911e..58ce62d643a 100755
--- a/types/generated-snapshot/experimental/index.ts
+++ b/types/generated-snapshot/experimental/index.ts
@@ -1756,7 +1756,7 @@ export interface MessageEventInit {
   data?: any;
   origin?: string;
   lastEventId?: string;
-  source?: MessagePort;
+  source?: MessagePort | null;
   ports?: MessagePort[];
 }
 /**
diff --git a/types/generated-snapshot/index.d.ts b/types/generated-snapshot/index.d.ts
index bf045f65615..aab5bb0dabc 100755
--- a/types/generated-snapshot/index.d.ts
+++ b/types/generated-snapshot/index.d.ts
@@ -1726,7 +1726,7 @@ interface MessageEventInit {
   data?: any;
   origin?: string;
   lastEventId?: string;
-  source?: MessagePort;
+  source?: MessagePort | null;
   ports?: MessagePort[];
 }
 /**
diff --git a/types/generated-snapshot/index.ts b/types/generated-snapshot/index.ts
index bf91bddca5a..f0c25c20e72 100755
--- a/types/generated-snapshot/index.ts
+++ b/types/generated-snapshot/index.ts
@@ -1730,7 +1730,7 @@ export interface MessageEventInit {
   data?: any;
   origin?: string;
   lastEventId?: string;
-  source?: MessagePort;
+  source?: MessagePort | null;
   ports?: MessagePort[];
 }
 /**