diff --git a/src/util/to-string.js b/src/util/to-string.js
index 669b983a..34e9fd5e 100644
--- a/src/util/to-string.js
+++ b/src/util/to-string.js
@@ -1,7 +1,7 @@
 import isBigInt from './is-bigint';
 
 export default function(v) {
-  return isBigInt(v)
-    ? (v.toString() + 'n')
+  return v === undefined ? v + ''
+    : isBigInt(v) ? v + 'n'
     : JSON.stringify(v);
 }
\ No newline at end of file