diff --git a/test/test-fwknop.pl b/test/test-fwknop.pl
index 30d1af8d..198f12e8 100755
--- a/test/test-fwknop.pl
+++ b/test/test-fwknop.pl
@@ -1386,8 +1386,8 @@ sub process_include_exclude() {
 
 sub diff_test_results() {
 
-    $diff_dir1 = "${output_dir}.last" unless $diff_dir2;
-    $diff_dir2 = $output_dir unless $diff_dir1;
+    $diff_dir1 = "${output_dir}.last" unless $diff_dir1;
+    $diff_dir2 = $output_dir unless $diff_dir2;
 
     die "[*] Need results from a previous run before running --diff"
         unless -d $diff_dir2;