diff --git a/ATxService/AutoTx.cs b/ATxService/AutoTx.cs
index 72534e90b9bf5e1def665e68a57ed3bf9a7a503a..cd7348bcef87f0d03dfe6ea76341a439318f6532 100644
--- a/ATxService/AutoTx.cs
+++ b/ATxService/AutoTx.cs
@@ -29,7 +29,7 @@ namespace ATxService
         private string _pathToConfig;
         private string _pathToStatus;
 
-        private List<string> _transferredFiles = new List<string>();
+        private readonly List<string> _transferredFiles = new List<string>();
 
         private int _txProgress;