diff --git a/test/index.js b/test/index.js
index e1c4753..c56ef71 100644
--- a/test/index.js
+++ b/test/index.js
@@ -40,7 +40,10 @@ test ('random_string with default length', (t) => {
 });
 
 test ('random_string should refuse lenght smaller 1', (t) => {
-  t.throws (() => (crypto.random_string (0)));
+  const error = t.throws (
+    () => (crypto.random_string (0))
+  );
+  t.is(error.message, 'invalid length');
 });
 
 test ('hash_sha512', (t) => {