diff --git a/src/reconciler.ts b/src/reconciler.ts
index 63e0c16..99809ee 100644
--- a/src/reconciler.ts
+++ b/src/reconciler.ts
@@ -454,7 +454,7 @@ export const reconciler = Reconciler<
 /**
  * Safely flush async effects when testing, simulating a legacy root.
  */
-export const act: Act = (React as any)?.unstable_act ?? (React as any)?.act
+export const act: Act = 'unstable_act' in React ? (React as any).unstable_act : (React as any).act
 
 // Inject renderer meta into devtools
 const isProd = typeof process === 'undefined' || process.env?.['NODE_ENV'] === 'production'