diff --git a/lib/util.js b/lib/util.js
index 20fe8f68..9b38ec8c 100644
--- a/lib/util.js
+++ b/lib/util.js
@@ -62,6 +62,7 @@ function fromSetString(aStr) {
   return aStr;
 }
 exports.fromSetString = supportsNullProto ? identity : fromSetString;
+exports.isProtoString = isProtoString;
 
 function isProtoString(s) {
   if (!s) {