diff --git a/src/UniTask.NetCoreSandbox/Program.cs b/src/UniTask.NetCoreSandbox/Program.cs
index 65a63e5..d982b12 100644
--- a/src/UniTask.NetCoreSandbox/Program.cs
+++ b/src/UniTask.NetCoreSandbox/Program.cs
@@ -16,8 +16,16 @@ namespace NetCoreSandbox
 
         static async UniTask<int> outer()
         {
-            var v = await DoAsync();
-            return v;
+            //await Task.WhenAll();
+
+            //var foo = await Task.WhenAny(Array.Empty<Task<int>>());
+
+
+            await UniTask.WhenAny(new UniTask[0]);
+
+            return 10;
+            //var v = await DoAsync();
+            //return v;
         }
 
 
diff --git a/src/UniTask/Assets/Plugins/UniTask/Internal/ArrayPoolUtil.cs b/src/UniTask/Assets/Plugins/UniTask/Internal/ArrayPoolUtil.cs
index 0892dc6..016901d 100644
--- a/src/UniTask/Assets/Plugins/UniTask/Internal/ArrayPoolUtil.cs
+++ b/src/UniTask/Assets/Plugins/UniTask/Internal/ArrayPoolUtil.cs
@@ -32,11 +32,21 @@ namespace Cysharp.Threading.Tasks.Internal
             }
         }
 
-        public static RentArray<T> CopyToRentArray<T>(IEnumerable<T> source)
+        public static RentArray<T> Materialize<T>(IEnumerable<T> source)
         {
+            if (source is T[] array)
+            {
+                return new RentArray<T>(array, array.Length, null);
+            }
+
             var defaultCount = 32;
             if (source is ICollection<T> coll)
             {
+                if (coll.Count == 0)
+                {
+                    return new RentArray<T>(Array.Empty<T>(), 0, null);
+                }
+
                 defaultCount = coll.Count;
                 var pool = ArrayPool<T>.Shared;
                 var buffer = pool.Rent(defaultCount);
diff --git a/src/UniTask/Assets/Plugins/UniTask/UniTask.WhenAll.cs b/src/UniTask/Assets/Plugins/UniTask/UniTask.WhenAll.cs
index 5f8f340..714da5b 100644
--- a/src/UniTask/Assets/Plugins/UniTask/UniTask.WhenAll.cs
+++ b/src/UniTask/Assets/Plugins/UniTask/UniTask.WhenAll.cs
@@ -11,12 +11,17 @@ namespace Cysharp.Threading.Tasks
     {
         public static UniTask<T[]> WhenAll<T>(params UniTask<T>[] tasks)
         {
+            if (tasks.Length == 0)
+            {
+                return UniTask.FromResult(Array.Empty<T>());
+            }
+
             return new UniTask<T[]>(new WhenAllPromise<T>(tasks, tasks.Length), 0);
         }
 
         public static UniTask<T[]> WhenAll<T>(IEnumerable<UniTask<T>> tasks)
         {
-            using (var span = ArrayPoolUtil.CopyToRentArray(tasks))
+            using (var span = ArrayPoolUtil.Materialize(tasks))
             {
                 var promise = new WhenAllPromise<T>(span.Array, span.Length); // consumed array in constructor.
                 return new UniTask<T[]>(promise, 0);
@@ -25,12 +30,17 @@ namespace Cysharp.Threading.Tasks
 
         public static UniTask WhenAll(params UniTask[] tasks)
         {
+            if (tasks.Length == 0)
+            {
+                return UniTask.CompletedTask;
+            }
+
             return new UniTask(new WhenAllPromise(tasks, tasks.Length), 0);
         }
 
         public static UniTask WhenAll(IEnumerable<UniTask> tasks)
         {
-            using (var span = ArrayPoolUtil.CopyToRentArray(tasks))
+            using (var span = ArrayPoolUtil.Materialize(tasks))
             {
                 var promise = new WhenAllPromise(span.Array, span.Length); // consumed array in constructor.
                 return new UniTask(promise, 0);
@@ -48,6 +58,14 @@ namespace Cysharp.Threading.Tasks
                 TaskTracker.TrackActiveTask(this, 3);
 
                 this.completeCount = 0;
+
+                if (tasksLength == 0)
+                {
+                    this.result = Array.Empty<T>();
+                    core.TrySetResult(result);
+                    return;
+                }
+
                 this.result = new T[tasksLength];
 
                 for (int i = 0; i < tasksLength; i++)
@@ -144,6 +162,12 @@ namespace Cysharp.Threading.Tasks
                 this.tasksLength = tasksLength;
                 this.completeCount = 0;
 
+                if (tasksLength == 0)
+                {
+                    core.TrySetResult(AsyncUnit.Default);
+                    return;
+                }
+
                 for (int i = 0; i < tasksLength; i++)
                 {
                     UniTask.Awaiter awaiter;
diff --git a/src/UniTask/Assets/Plugins/UniTask/UniTask.WhenAny.cs b/src/UniTask/Assets/Plugins/UniTask/UniTask.WhenAny.cs
index 64de31c..15ecd1f 100644
--- a/src/UniTask/Assets/Plugins/UniTask/UniTask.WhenAny.cs
+++ b/src/UniTask/Assets/Plugins/UniTask/UniTask.WhenAny.cs
@@ -21,7 +21,7 @@ namespace Cysharp.Threading.Tasks
 
         public static UniTask<(int winArgumentIndex, T result)> WhenAny<T>(IEnumerable<UniTask<T>> tasks)
         {
-            using (var span = ArrayPoolUtil.CopyToRentArray(tasks))
+            using (var span = ArrayPoolUtil.Materialize(tasks))
             {
                 return new UniTask<(int, T)>(new WhenAnyPromise<T>(span.Array, span.Length), 0);
             }
@@ -36,7 +36,7 @@ namespace Cysharp.Threading.Tasks
         /// <summary>Return value is winArgumentIndex</summary>
         public static UniTask<int> WhenAny(IEnumerable<UniTask> tasks)
         {
-            using (var span = ArrayPoolUtil.CopyToRentArray(tasks))
+            using (var span = ArrayPoolUtil.Materialize(tasks))
             {
                 return new UniTask<int>(new WhenAnyPromise(span.Array, span.Length), 0);
             }
@@ -186,6 +186,11 @@ namespace Cysharp.Threading.Tasks
 
             public WhenAnyPromise(UniTask<T>[] tasks, int tasksLength)
             {
+                if (tasksLength == 0)
+                {
+                    throw new ArgumentException("The tasks argument contains no tasks.");
+                }
+
                 TaskTracker.TrackActiveTask(this, 3);
 
                 for (int i = 0; i < tasksLength; i++)
@@ -277,6 +282,11 @@ namespace Cysharp.Threading.Tasks
 
             public WhenAnyPromise(UniTask[] tasks, int tasksLength)
             {
+                if (tasksLength == 0)
+                {
+                    throw new ArgumentException("The tasks argument contains no tasks.");
+                }
+
                 TaskTracker.TrackActiveTask(this, 3);
 
                 for (int i = 0; i < tasksLength; i++)