diff --git a/tests/Smdn.Net.MuninNode/Smdn.Net.MuninNode/StreamReaderWriterExtensions.cs b/tests/Smdn.Net.MuninNode/Smdn.Net.MuninNode/StreamReaderWriterExtensions.cs
index dff7d14..1d73ac2 100644
--- a/tests/Smdn.Net.MuninNode/Smdn.Net.MuninNode/StreamReaderWriterExtensions.cs
+++ b/tests/Smdn.Net.MuninNode/Smdn.Net.MuninNode/StreamReaderWriterExtensions.cs
@@ -24,6 +24,8 @@ public static async Task WriteLineAsync(this StreamWriter writer, string value,
     => await writer.WriteLineAsync(value.AsMemory()).WaitAsync(cancellationToken).ConfigureAwait(false);
 #endif
 
+#if !SYSTEM_IO_TEXTWRITER_FLUSHASYNC_CANCELLATIONTOKEN
   public static async Task FlushAsync(this StreamWriter writer, CancellationToken cancellationToken)
     => await writer.FlushAsync(cancellationToken).WaitAsync(cancellationToken).ConfigureAwait(false);
+#endif
 }