diff --git a/src/benchmarks/micro/libraries/System.IO.FileSystem/Perf.FileStream.cs b/src/benchmarks/micro/libraries/System.IO.FileSystem/Perf.FileStream.cs
index 32f7151955f..7da28e38024 100644
--- a/src/benchmarks/micro/libraries/System.IO.FileSystem/Perf.FileStream.cs
+++ b/src/benchmarks/micro/libraries/System.IO.FileSystem/Perf.FileStream.cs
@@ -72,6 +72,9 @@ public bool OpenClose(long fileSize, FileOptions options)
         [Arguments(OneKibibyte , FileOptions.None)]
         [Arguments(OneKibibyte , FileOptions.Asynchronous)]
         [AllowedOperatingSystems("Lock and Unlock are supported only on Windows and Linux", OS.Linux, OS.Windows)]
+#if NET6_0_OR_GREATER // the method was marked as unsupported on macOS in .NET 6.0
+        [System.Runtime.Versioning.UnsupportedOSPlatform("macos")]
+#endif
         public void LockUnlock(long fileSize, FileOptions options)
         {
             string filePath = _sourceFilePaths[fileSize];
diff --git a/src/benchmarks/micro/libraries/System.IO/NullWriteStream.cs b/src/benchmarks/micro/libraries/System.IO/NullWriteStream.cs
index b0fe2238ecc..4bc6adba6cd 100644
--- a/src/benchmarks/micro/libraries/System.IO/NullWriteStream.cs
+++ b/src/benchmarks/micro/libraries/System.IO/NullWriteStream.cs
@@ -41,7 +41,7 @@ public override void WriteByte(byte value) { }
 #if NETCOREAPP2_1_OR_GREATER // these virtual methods only exist in .NET Core 2.1+
         public override void Write(ReadOnlySpan<byte> buffer) { }
 
-        public override ValueTask WriteAsync(ReadOnlyMemory<byte> buffer, CancellationToken cancellationToken = default) => ValueTask.CompletedTask;
+        public override ValueTask WriteAsync(ReadOnlyMemory<byte> buffer, CancellationToken cancellationToken = default) => new ValueTask(Task.CompletedTask);
 #endif
     }
 }