diff --git a/src/filo-fs-nullfs.adb b/src/filo-fs-nullfs.adb
index 105e985..dc0ca28 100644
--- a/src/filo-fs-nullfs.adb
+++ b/src/filo-fs-nullfs.adb
@@ -64,7 +64,7 @@
    is
       use FILO.FS;
       Buffer : Buffer_Type (0 .. Integer(len) - 1) with Address => buf;
-      Offset : Natural := Natural(File_Pos);
+      Offset : constant Natural := Natural(File_Pos);
       Success : Boolean;
    begin
       Read (Global_NullFS, Buffer, Offset, Success);