diff --git a/toolkit/components/url-classifier/content/url-crypto-key-manager.js b/toolkit/components/url-classifier/content/url-crypto-key-manager.js
index 90db4862350..943319d0da1 100644
--- a/toolkit/components/url-classifier/content/url-crypto-key-manager.js
+++ b/toolkit/components/url-classifier/content/url-crypto-key-manager.js
@@ -260,7 +260,7 @@ PROT_UrlCryptoKeyManager.prototype.hasKey = function() {
 
 PROT_UrlCryptoKeyManager.prototype.unUrlSafe = function(key)
 {
-    return key.replace("-", "+").replace("_", "/");
+    return key ? key.replace("-", "+").replace("_", "/") : "";
 }
 
 /**