Fixes #1

src/AsyncEnumerator/AsyncEnumerator.cs

@@ -18,7 +18,7 @@ public class AsyncEnumerator<T> : TaskLikeBase, IAsyncEnumeratorProducer<T>, IAs
         private ExceptionDispatchInfo _exception;
 
         private bool _isStarted;
-        private TaskCompletionSource<bool> _nextSource;
+        private TaskCompletionSource<bool> _nextSource =new TaskCompletionSource<bool>();
         private TaskCompletionSource<bool> _yieldSource;
 
         public static TaskProvider<IAsyncEnumeratorProducer<T>> Capture() => TaskProvider<IAsyncEnumeratorProducer<T>>.Instance;
@@ -32,7 +32,7 @@ public Task<bool> MoveNextAsync()
             if (!_isStarted)
             {
                 _isStarted = true;
-                return Task.FromResult(true);
+                return _nextSource.Task;
             }
 
             _nextSource = new TaskCompletionSource<bool>();
@@ -45,7 +45,7 @@ public Task<bool> MoveNextAsync()
         internal override void SetException(ExceptionDispatchInfo exception)
         {
             _exception = exception;
-            _nextSource?.TrySetException(exception.SourceException);
+            _nextSource.TrySetException(exception.SourceException);
         }
 
         T IAsyncEnumeratorProducer<T>.Break()
@@ -65,10 +65,8 @@ Task IAsyncEnumeratorProducer<T>.Pause()
         Task IAsyncEnumeratorProducer<T>.Return(T value)
         {
             Current = value;
-
             _yieldSource = new TaskCompletionSource<bool>();
-
-            _nextSource?.TrySetResult(true);
+            _nextSource.TrySetResult(true);
 
             return _yieldSource.Task;
         }

src/AsyncEnumeratorTests/AsyncEnumeratorTests.cs

@@ -54,6 +54,15 @@ public async Task EnumerationAdvancesCorrectlyAndCompletes2()
             Assert.IsTrue(seq.IsCompleted, "Enumeration did not complete after return.");
         }
 
+        [TestMethod]
+        public async Task EmptyEnumeratorTest()
+        {
+            var seq = GetEmptyEnumerator();
+
+            Assert.IsFalse(await seq.MoveNextAsync(), $"Call to {nameof(seq.MoveNextAsync)} did not return false after enumeration completed.");
+
+            Assert.IsTrue(seq.IsCompleted, "Enumeration did not complete after return.");
+        }
 
         private static async AsyncEnumerator<int> ExceptionTest1()
         {
@@ -89,5 +98,12 @@ private static async AsyncEnumerator<int> Test2()
             return yield.Break();
         }
 
+        private static async AsyncEnumerator<int> GetEmptyEnumerator()
+        {
+            var yield = await AsyncEnumerator<int>.Capture();
+
+            return yield.Break();
+        }
+
     }
 }

src/AsyncEnumeratorTests/AsyncSequenceTests.cs

@@ -54,6 +54,16 @@ public async Task EnumerationAdvancesCorrectlyAndCompletes2()
             Assert.IsTrue(seq.IsCompleted, "Enumeration did not complete after return.");
         }
 
+        [TestMethod]
+        public async Task EmptyEnumeratorTest()
+        {
+            var seq = GetEmptyEnumerator();
+
+            Assert.IsFalse(await seq.MoveNextAsync(), $"Call to {nameof(seq.MoveNextAsync)} did not return false after enumeration completed.");
+
+            Assert.IsTrue(seq.IsCompleted, "Enumeration did not complete after return.");
+        }
+
         private static async AsyncEnumerator<int> ExceptionTest1()
         {
             var yield = await AsyncEnumerator<int>.Capture();
@@ -91,5 +101,12 @@ private static async AsyncSequence<int> Test2()
 
             return yield.Break();
         }
+
+        private static async AsyncSequence<int> GetEmptyEnumerator()
+        {
+            var yield = await AsyncSequence<int>.Capture();
+
+            return yield.Break();
+        }
     }
 }