diff --git a/Assets/UniRx.Async/UnityAsyncExtensions.cs b/Assets/UniRx.Async/UnityAsyncExtensions.cs
index 77fe595..fdb6cd3 100644
--- a/Assets/UniRx.Async/UnityAsyncExtensions.cs
+++ b/Assets/UniRx.Async/UnityAsyncExtensions.cs
@@ -792,6 +792,11 @@ namespace UniRx.Async
 
             public void UnsafeOnCompleted(Action continuation)
             {
+                if(asyncOperation != null && asyncOperation.isDone)
+                {
+                    continuation?.Invoke();
+                    return;
+                }
                 Error.ThrowWhenContinuationIsAlreadyRegistered(continuationAction);
                 continuationAction = continuation.AsFuncOfT<AsyncOperation>();
                 asyncOperation.completed += continuationAction;