diff --git a/tests/functional/lang/eval-okay-substring.exp b/tests/functional/lang/eval-okay-substring.exp
index 6aace04b0..f48b4623a 100644
--- a/tests/functional/lang/eval-okay-substring.exp
+++ b/tests/functional/lang/eval-okay-substring.exp
@@ -1 +1 @@
-"ooxfoobarybarzobaabbc"
+"ooxfoobarybarzobaabbc_bad"
diff --git a/tests/functional/lang/eval-okay-substring.nix b/tests/functional/lang/eval-okay-substring.nix
index 424af00d9..54c97e162 100644
--- a/tests/functional/lang/eval-okay-substring.nix
+++ b/tests/functional/lang/eval-okay-substring.nix
@@ -19,3 +19,5 @@ substring 1 2 s
 + substring 3 1 s
 + "c"
 + substring 5 10 "perl"
++ "_"
++ substring 3 (-1) "tebbad"