diff --git a/src/ICSharpCode.SharpZipLib/GZip/GzipOutputStream.cs b/src/ICSharpCode.SharpZipLib/GZip/GzipOutputStream.cs
index 83b0e489..65a9e33e 100644
--- a/src/ICSharpCode.SharpZipLib/GZip/GzipOutputStream.cs
+++ b/src/ICSharpCode.SharpZipLib/GZip/GzipOutputStream.cs
@@ -242,7 +242,7 @@ public override void Finish()
 
 		#region Support Routines
 
-		private string CleanFilename(string path)
+		private static string CleanFilename(string path)
 			=> path.Substring(path.LastIndexOf('/') + 1);
 
 		private void WriteHeader()