diff --git a/lib/cli/run.js b/lib/cli/run.js
index 6eb5ed9a49..92908be435 100644
--- a/lib/cli/run.js
+++ b/lib/cli/run.js
@@ -369,7 +369,7 @@ exports.handler = async function (argv) {
   try {
     await runMocha(mocha, argv);
   } catch (err) {
-    if (!err) {
+    if (!err || !err.toString) {
       console.error('\n Undefined error:', err);
     } else {
       console.error('\n' + (err.stack || `Error: ${err.message || err}`));