diff --git a/helper/tt2-process b/helper/tt2-process
index 0631c74e..fdf9fb75 100755
--- a/helper/tt2-process
+++ b/helper/tt2-process
@@ -295,7 +295,7 @@ sub generate_iofiles {
     my @filenames = map { {
         input => $_,
         output => $filenames{$_},
-    } } (sort @filenames_prio, sort @filenames_norm);
+    } } (sort(@filenames_prio), sort(@filenames_norm));
 
     return @filenames;
 }