diff --git a/webview/shared/src/chat/lib/messageHandler.test.ts b/webview/shared/src/chat/lib/messageHandler.test.ts
index 4bc7fe9..90e4e99 100644
--- a/webview/shared/src/chat/lib/messageHandler.test.ts
+++ b/webview/shared/src/chat/lib/messageHandler.test.ts
@@ -73,6 +73,25 @@ describe('extractEventMessageId', () => {
   });
 });
 
+describe('malformed stream events', () => {
+  it('ignores null, undefined, primitive, and array event payloads', () => {
+    let state = { ...initialState, currentSessionId: 'ses-malformed' } as AppState;
+    const dispatch = (action: Parameters<typeof appReducer>[1]) => {
+      state = appReducer(state, action);
+    };
+    const handler = createMessageHandler(dispatch, () => state);
+
+    for (const event of [null, undefined, 'invalid', []]) {
+      handler({
+        data: { type: 'streamEvent', sessionId: 'ses-malformed', event },
+      } as MessageEvent);
+    }
+
+    assert.equal(state.isProcessing, false);
+    assert.equal(state.streaming, null);
+  });
+});
+
 describe('normalizeMessage - responseType handling', () => {
   it('ignores undefined interactive events from an untrusted payload', () => {
     const result = normalizeMessage({
diff --git a/webview/shared/src/chat/lib/messageHandler.ts b/webview/shared/src/chat/lib/messageHandler.ts
index 1c14aa2..999fabb 100644
--- a/webview/shared/src/chat/lib/messageHandler.ts
+++ b/webview/shared/src/chat/lib/messageHandler.ts
@@ -9251,6 +9251,8 @@ function handleStreamEvent(
   knownReasoningPartIDs?: Set<string>,
   pendingRenderableTextPart?: { partID?: string; messageID?: string },
 ): void {
+  if (!payload || typeof payload !== "object") return;
+
   const dispatchProcessingTrue = () => {
     if (!shouldSuppressProcessingBootstrap) {
       dispatch({ type: "SET_IS_PROCESSING", payload: true });
@@ -14303,7 +14305,11 @@ export function createMessageHandler(dispatch: Dispatch<AppAction>, getState: ()
         }
         case "streamEvent": {
           const stateBeforeStreamEvent = getState();
-          const payload = asRecord(data.event) ?? data;
+          const payload = asRecord(data.event);
+          if (!payload || Array.isArray(payload)) {
+            logger.warn("Ignoring stream event with an invalid payload");
+            break;
+          }
           const streamEventType = asString(payload.type) || "unknown";
           const envelopeSessionId =
             asString(data.sessionId) ||