diff --git a/packages/wxt/src/utils/__tests__/content-script-context.test.ts b/packages/wxt/src/utils/__tests__/content-script-context.test.ts
index f7a5ffc8..ca7bdcac 100644
--- a/packages/wxt/src/utils/__tests__/content-script-context.test.ts
+++ b/packages/wxt/src/utils/__tests__/content-script-context.test.ts
@@ -32,6 +32,25 @@ describe('Content Script Context', () => {
     expect(isValid).toBe(false);
   });
 
+  it('should not throw when browser.runtime is undefined (extension context fully invalidated)', () => {
+    const ctx = new ContentScriptContext('test');
+    const onInvalidated = vi.fn();
+
+    ctx.onInvalidated(onInvalidated);
+    // Simulate complete extension context invalidation where browser.runtime becomes undefined
+    const originalRuntime = fakeBrowser.runtime;
+    // @ts-ignore
+    fakeBrowser.runtime = undefined;
+
+    // Should not throw, and should mark as invalid
+    expect(() => ctx.isInvalid).not.toThrow();
+    expect(ctx.isInvalid).toBe(true);
+    expect(onInvalidated).toBeCalled();
+
+    // Restore for other tests
+    fakeBrowser.runtime = originalRuntime;
+  });
+
   it('should invalidate the current content script when a new context is created', async () => {
     const name = 'test';
     const onInvalidated = vi.fn();
diff --git a/packages/wxt/src/utils/content-script-context.ts b/packages/wxt/src/utils/content-script-context.ts
index fdcbe857..7fa84da0 100644
--- a/packages/wxt/src/utils/content-script-context.ts
+++ b/packages/wxt/src/utils/content-script-context.ts
@@ -71,7 +71,7 @@ export class ContentScriptContext implements AbortController {
   }
 
   get isInvalid(): boolean {
-    if (browser.runtime.id == null) {
+    if (browser.runtime?.id == null) {
       this.notifyInvalidated(); // Sets `signal.aborted` to true
     }
     return this.signal.aborted;