diff --git a/lib/extensions.js b/lib/extensions.js
index 48afae4c..7b086554 100644
--- a/lib/extensions.js
+++ b/lib/extensions.js
@@ -107,7 +107,7 @@ module.exports = function(realmConstructor) {
 
         if (realmConstructor.Sync._setFeatureToken) {
             realmConstructor.Sync.setFeatureToken = function(featureToken) {
-                if (typeof featureToken !== 'string' || !(featureToken instanceof String)) {
+                if (typeof featureToken !== 'string' && !(featureToken instanceof String)) {
                     throw new Error("featureToken should be a string");
                 }